molecular formation via grain-surface reactions

UNIVERSITÀ DEGLI STUDI DI CAGLIARIFacoltà di Scienze Matematiche, Fisiche e NaturaliDipartimento di FisicaCagliari, ITALYPhD Dissertationin PhysicsEXPERIMENTAL ASTROCHEMISTRY:MOLECULAR FORMATION VIAGRAIN-SURFACE REACTIONSbyEMANUELE CONGIU

Supervisor:Prof. Valerio PirronelloUniversity of Catania, Sicily, Italy

This work is dedicated to a young manthat used to be eager to succeed.He eventually made it.

Typeset in L A TEX by the author.Copyright c○ E. Congiu. All Rights Reserved.2006

Contents1 Interstellar dust, gas, and chemistry 11.1 Interstellar dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.1 The key roles of dust grains in the ISM . . . . . . . . . . . . . . 41.2 Interstellar gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3 Astrochemistry... a matter of molecules . . . . . . . . . . . . . . . . . . 71.3.1 Gas-phase chemistry . . . . . . . . . . . . . . . . . . . . . . . . 81.3.2 Grain-surface chemistry . . . . . . . . . . . . . . . . . . . . . . 91.4 Molecular hydrogen in space . . . . . . . . . . . . . . . . . . . . . . . . 101.4.1 Astrophysical importance of H 2 . . . . . . . . . . . . . . . . . . 101.4.2 Mechanisms of H 2 formation on dust grains . . . . . . . . . . . 121.5 Laboratory experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 141.6 This thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 The Laboratory ofSurface Science at SU 192.1 Overview of laboratory equipment . . . . . . . . . . . . . . . . . . . . . 192.1.1 Beamlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.1.2 Main chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.1.3 Data acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.2.1 Theoretical considerations . . . . . . . . . . . . . . . . . . . . . 242.2.2 Importance of the two-beamline configuration . . . . . . . . . . 292.2.3 Beam intensities . . . . . . . . . . . . . . . . . . . . . . . . . . 292.2.4 Adsorption and TPD procedures . . . . . . . . . . . . . . . . . 302.2.5 Desorption yield and recombination efficiency calculations . . . 322.2.6 Typical protocol of measurements . . . . . . . . . . . . . . . . . 343 Molecular Hydrogen Formation on Amorphous Olivine 373.1 A brief survey of previous studies and experiments . . . . . . . . . . . 383.1.1 Pre-Syracuse era . . . . . . . . . . . . . . . . . . . . . . . . . . 383.1.2 Syracuse era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2 Experiments on amorphous olivine . . . . . . . . . . . . . . . . . . . . 46

viiiContents3.2.1 Description of the samples employed . . . . . . . . . . . . . . . 463.2.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . 493.2.3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . 503.3 Preliminary fit of the experimental results . . . . . . . . . . . . . . . . 593.3.1 The model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.3.2 A preliminary fit . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4 Summary and implications of astrophysicalrelevance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634 Construction of the FT-RAIRS facility 674.1 IR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.2 FT-IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.2.1 A brief review of the Fourier Transform methods . . . . . . . . 714.2.2 Principles of FT-IR: The Michelson Interferometer . . . . . . . . 724.2.3 Advantages of FT-IR instruments over dispersive instruments . 754.2.4 Wavelength accuracy through the HeNe laser . . . . . . . . . . . 764.3 The RAIRS technique . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.3.1 Physical principles . . . . . . . . . . . . . . . . . . . . . . . . . 784.3.2 Further remarks about RAIRS . . . . . . . . . . . . . . . . . . . 804.4 Importing FT-RAIRS into our laboratory . . . . . . . . . . . . . . . . . 814.4.1 Preliminary study of the project . . . . . . . . . . . . . . . . . . 814.4.2 The choice of the 78 ◦ incident angle and overview of the newconfiguration proposal . . . . . . . . . . . . . . . . . . . . . . . 844.5 The Nicolet TM FT-IR 6700 Spectrometer . . . . . . . . . . . . . . . . . 864.5.1 IR source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.6 The MCT detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874.7 The custom 8 - 3 3/8 inch coupling flanges and the DPWs . . . . . . . 894.8 The custom-made off-axis mirrors . . . . . . . . . . . . . . . . . . . . . 914.9 The final FT-RAIRS setup . . . . . . . . . . . . . . . . . . . . . . . . . 935 Summary and conclusions 1015.1 Astrochemistry - Molecules in the ISM . . . . . . . . . . . . . . . . . . 1015.2 Grain-surface reactions and H 2 . . . . . . . . . . . . . . . . . . . . . . . 1025.3 Experimental apparatus at SU . . . . . . . . . . . . . . . . . . . . . . . 1025.4 H 2 formation on amorphous olivine . . . . . . . . . . . . . . . . . . . . 1045.5 Construction of the FT-RAIRS facility . . . . . . . . . . . . . . . . . . 1055.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.7 Future projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Chapter 1Interstellar dust, gas, andchemistryMost of the mass of our Galaxy, the Milky Way, is concentrated in stars. Besides, allthe stars with their possible planetary systems in the Galaxy hardly fill a fraction ofa billionth of its volume at the very most. The space in the remaining volume is nottotally empty, though. It contains matter in the form of dust and gas to which werefer as interstellar medium (ISM). This component is inhomogeneously spread outacross the interstellar space and it is mostly located near the Galactic plane and alongthe spiral arms of the Milky Way.The ISM accounts for 10-15% of the total mass of the Galaxy: approximately 99%of the mass of the ISM is in the form of gas, so only the remainder 1% in mass is inthe form of dust grains, but we shall see below how important dust proves to be. Onaverage, the interstellar medium has a density of 1 H atom/cm 3 .The chemical composition of interstellar matter is close to the so called cosmiccomposition inferred from element abundances in the Sun. The gas is almost entirelyhydrogen, with about 10% of helium by number; oxygen, carbon and nitrogencomprise about 0.1% by number, and all other elements are present only in tracequantities.The ISM is extremely important in astrophysics for the role it plays in the lifecycleof stars, and, consequently, in the overall evolution of the galaxies (Tielens2005, Dyson & Williams 1997, Ferrière 2001); stars, in fact, form in the densest andcoldest regions of the ISM, after these interstellar clouds of dust and gas becomegravitationally unstable and begin to collapse. Within the interior of stars, part ofthe former interstellar matter (mostly H and He), is enriched with heavier elementsby nucleosynthesis. In this way, the abundances of heavy element in the Universeincrease. Then this matter returns to the ISM via stellar winds and supernovaeexplosions, ready to serve as a birthplace of future generations of stars.However, the ISM cannot be considered as a mere substrate where stars originate,evolve and die, but it proves to be a primary active element in the Galaxy. It is

2 Chapter 1. Interstellar dust, gas, and chemistrycharacterised by important energy sources such as gravitational potential energy ofthe interstellar clouds, strong ultraviolet radiation fields, cosmic rays, shock wavesgenerated by supernovae explosions, stellar outflows, and collisions between denseclouds. In this way, the spatial, physical and chemical characteristics of the interstellarmedium strongly affect mass and luminosity of stars which, in turn, determinethe overall structure and the large-scale dynamics of the Galaxy. Therefore, a deepunderstanding of the dynamics, physics and chemistry of the ISM is a must in thewider landscape of astrophysical issues.1.1 Interstellar dustDust grains are a ubiquitous component of the ISM . Evidence for the existenceof cosmic dust is actually very clear: its most visible manifestation is the generalobscuration of starlight throughout the Milky Way (and in other galaxies too). Acomparison of the expected intensity of starlight from a star at a known distancewith that actually detected shows that the measured emission is always less thanexpected. The difference is attributed to interstellar extinction by dust, namely, tothe combined effects of absorption, and scattering, whose principal effects can bedescribed by the Lorentz-Mie theory for the scattering of electromagnetic radiationby spherical particles. Interstellar extinction measured in this way is found to be smallin the infrared, to rise almost linearly in the visible regions of the spectrum, to havea peak in the near ultraviolet at a wavelength around 220 nm, then to rise stronglyin the far UV, at least down to wavelengths as short as 100 nm. The precise shapevaries along different lines of sight (Savage & Mathis 1979, Draine 2003), though thegeneral character remains the same (see Fig. 1.1). The near-linear rise in the visiblepart of the spectrum implies the existence of some dust grains that are of a sizecomparable with the wavelength of visible light, i.e. about 500 nm, and the strongrise in the far UV shows that many smaller grains must be present as well. Thesize distribution of interstellar grains is known to have a dependence on the radiusr raised to the power -3.5; namely, there are many more smaller grains than largerones (Mathis et al. 1977). Depending on their size, dust grains can be referred toas: standard grains, very small grains, and, at the smallest sizes, PAH 1 -like speciescomposed of aromatic molecules but possibly less hydrogenic in space than laboratoryPAHs (Weingartner & Draine 2001, Bakes et al. 2001). Some typical ranges of grainsizes are: 0.03 - 3 µm for standard grains, 2-10 nm for very small grains, and 0.5 -1.0 nm for PAHs. Of course, grains are far from being spherical; it is more likely thatgrains are asymmetric spheroids with a fluffy form (Mathis 1998, Iatì et al. 2001). Thebroad peak at a wavelength of 220 nm is usually ascribed to small graphite particles,or to other carbonaceous materials.1 Polycyclic Aromatic Hydrocarbon.

1.1. Interstellar dust 3Figure 1.1: Set of extinction curves in function of the parameter R V = A V /E B−V (totalto-selectiveextinction ratio) that characterises the region that produces the extinction. Thesmaller R V , the more small grains will be present, causing a high extinction in the UV, andvice versa. From Draine (2003)Another effect due to submicron-sized solid dust particles is that the extinction ofstarlight is more effective at shorter wavelengths, which are closer to the typical grainsize, so that blue light is more severely dimmed than red light. That is why a starappears to us redder that it actually is. This effect is called reddening of starlight.Interstellar linear polarization (typically a few percent) is also detected in thestarlight, and is interpreted as differential extinction through a medium in whichasymmetric dust grains are partially aligned. Thus one plane of polarization is moreheavily extinguished than another. Therefore, the detection of linear polarizationis also evidence of the presence of dust grains, and that they must be in generalasymmetric.On average, the dust column density appears closely correlated with the hydrogencolumn density (Jenkins & Savage 1974, Bohlin 1975), thereby providing proof thatinterstellar dust tends to follow the inhomogeneous distribution of the interstellar gaswith the mean mass ratio of dust to gas being ∼ 0.01 (Pei 1992); although severalcases of reduced dust to gas ratio were found (Congiu et al. 2005).More specific information as to the composition of cosmic dust can be obtainedfrom considerations of elemental depletions. In fact, compared to a set of cosmicabundances (such as in the Sun), many heavier elements are significantly depleted inthe interstellar gas phase, indicating that these elements may have condensed into

4 Chapter 1. Interstellar dust, gas, and chemistrysolid dust grains (Snow & Witt 1996). Though considerations of depletion suggestthat dust is certainly a mixture of at least several elements, it is amazing that theseconsiderations are consistent with the general conclusions from the study of interstellarextinction: the dust in low density regions of the ISM is mainly various types ofsilicates and carbons. On the other hand, the dust grains in dense clouds are coatedwith mantles of water ice. This water ice is not pure, but contains CO molecules(detected by their C-O stretch mode at 4.7 µm), CO 2 molecules (4.3 µm), and severalother species, such as methanol, formic acid and methane, that freeze out onto thegrains. Thus the chemical complexity of dust in dense clouds is clearly greater thanin low density regions.1.1.1 The key roles of dust grains in the ISMDust grains are no longer considered simply as a component inhibiting the visibilityof stars and galaxies; dust is now recognized as playing a fundamental role in theevolution and current state of the Universe.As we have seen earlier, the most obvious and apparent effect of dust grains isthe extinction of starlight. This also means that regions of space are shielded fromstarlight, the photo-dissociation of molecules in those regions is hindered, and thus arich chemistry becomes possible. The molecules which are not destroyed, and the oneswhich are formed, are important coolants (they extract kinetic energy from the gasand lose it via radiative emissions), affecting the dynamical and thermal evolutionof the gas in star formation. Dust grains too, besides molecules, radiate at longwavelengths the UV and optical radiation they have absorbed, and thus contribute tothe cooling. Therefore, dust strongly affects the star formation process.In regions interpenetrated by starlight, the photoelectric effect upon dust grainsgenerates energetic electrons that are the main heating source; hence, dust is the mosteffective means of coupling the energy in the starlight into the interstellar gas.Dust grains, by removing from the gas the materials of which they are composed,make some elements (e.g. Fe, Si, and Ni) very rare in the gas phase. So these depletedspecies contribute less to the chemistry or to the level of ionisation than there wouldhave been otherwise.Moreover, the surfaces of dust grains are chemical catalysts, too. Our concernin this work is the relation and interaction between the gas and the dust, and theconsequent surface chemistry involved. We shall describe below the experimental evidence(Chapter 3) that interstellar molecular hydrogen is formed efficiently in surfacereaction on dust grains, and we shall note that nearly all of interstellar chemistry dependson the presence of H 2 . In this sense, dust grains may be considered an essentialcomponent of the present Universe, and we can assume that star formation must haveevolved differently before dust grains were abundant. Of course, other species than

1.2. Interstellar gas 5H 2 , too, must form similarly in reactions at the surfaces of dust grains; in low densityregions of the ISM these species may be mainly simple hydrides, while in denser regionsthe dust grains provide a substrate on which molecular ices can form. These icesremove molecules other than molecular hydrogen from the gas phase, and evidencefor gas depleted of its heavy molecules is common. Ices can also act as reservoirs ofmolecules that can be released back to the gas phase in warmer regions.1.2 Interstellar gasCosmic dust and the cooler, denser interstellar gas are intimately mixed. The gasclouds in which the cooler (T 10 2 K) and the denser (n H > 10 2 cm −3 ) gas is locatedoccupy only a few percent of the interstellar volume, but account for almost all of theinterstellar mass. Before discussing these denser and cooler regions, for the sake ofcompleteness I shall briefly describe other important components of interstellar gasthat fill the interstellar volume, but that are of little interest for the subject coveredin this dissertation.The coronal gas: occupies much of the interstellar volume; it is so-called becauseit has a very high temperature (∼ 10 6 K), comparable to that of the solar corona.It is shock-heated by the enormously energetic winds of supernovae and is detectedin X ray lines of highly ionised atoms of O VI and N V. The proton density of thecoronal gas is very low, 10 −2 cm −3 . Dust grains in such gas are rapidly destroyedby sputtering.H II regions: hot stars inside otherwise neutral regions maintain volumes of fullyionised gas around them. These ionised nebulae, or H II regions, have relatively hightemperatures, ∼ 10 4 K, and the density is that of the neutral region around them,generally ∼ 10 2 cm −3 . These interesting objects occupy only a small volume of space.In general, dust tends to be eroded in such regions.Another component of interstellar gas is the intercloud gas. It is atomic and maybe quite warm (∼ 10 3 K) and of low density (< 1 H atom cm −3 ). It is heated andpartially ionised by X rays from hot stars. It only comprises a small percentage ofthe interstellar mass.The main regions where gas and dust coexist are almost entirely neutral and cool.They are categorised as diffuse clouds (typical n H ∼ 50 cm −3 and T ∼ 50 − 100 K)and molecular clouds (typical n H ∼ 10 2 − 10 3 cm −3 and T ∼ 10 K), with a thirdcategory named translucent clouds that describes material of intermediate properties.In diffuse clouds the mean temperature of dust grains is around 15 - 20 K (Greenberg& Li 1996), and their number density is low enough that starlight can readily getthrough diffuse regions. UV and visible light from stars can then penetrate and ionisethe minor elements of carbon, sulfur and trace metals such as iron, silicon, magnesium,calcium, nickel and sodium. In diffuse clouds photo-dissociation and photo-ionisation

6 Chapter 1. Interstellar dust, gas, and chemistryare important in reducing the concentrations of molecules. Indeed, the gas consistsmainly of atomic material except for the case of hydrogen, by far the most abundantelement, which is roughly equally divided into neutral atoms and molecules. Molecularhydrogen has been observed in its far-UV absorption lines in diffuse interstellar cloudswith Copernicus (Jura 1975), and with FUSE (Shull et al. 2000). These studies haveshown that hydrogen in molecular form is dominant for column densities larger than2×10 20 H-atoms cm −2 .The high abundance of H 2 in diffuse clouds entails the presence of an active grainsurfacechemistry which initiates a chemical network that leads to the formation ofmore complex molecules. In fact, small abundances of other diatomic molecules (CO,CH, CH + , OH, NH, C 2 , CN, CS, and N 2 ) and a couple of triatomic species (H + 3 , C 3 )have been detected in infrared, visible and UV absorption spectra (Le Petit et al.2004), while many other have been sought unsuccessfully. The dust in diffuse regionsappears to be a a mixture of amorphous silicates and carbonaceous material, either asseparate grains or combined into more complex composites of a large number of verysmall subunits (Draine 2003, Li & Draine 2001, Weingartner & Draine 2001, Mathis1996).Molecular clouds are dense enough that they are opaque to starlight. As theirname implies, molecular clouds are almost entirely molecular. An active chemistryis required to maintain the detected relatively high abundances of these molecules.Because the intensity of the far-UV radiation field is lower within the molecular clouds,the gas-phase chemistry is primarily driven by cosmic rays, and a wealth of grainsurfacereactions is present too.The percentage of the material in dust particles is estimated to be about 1% bymass, a figure similar to that in diffuse clouds. Virtually all of the element hydrogenhas been converted into molecular form, although a small but detectable amount ofatomic hydrogen remains (Li & Goldsmith 2003).In these molecular clouds the material is shielded so well that complex moleculescan form, and conditions may be such that molecules can freeze out on the surface ofdust. Dust grains are thus very likely to be covered with icy mantles. These mantlesarise from a combination of chemistry occurring on the grain surfaces and accretionof molecules from the gas. Mantles of up to 100 monolayers have been observed, eventhough the size distribution of these core-mantle particles is highly uncertain.When molecular clouds are irradiated by nearby stars, the molecules in the regioncloser to the radiation source are photo-dissociated. These regions are called photodissociationregions (PDRs), and are a part of the molecular cloud in which photonsdominate the thermal and chemical balance of the gas. In PDRs, molecular hydrogenis photo-dissociated at low optical depths, namely, in the part closer to the star. Athigher optical depths, H 2 can form on dust grains, where UV rays are too weak todissociate these newly formed molecules.

1.3. Astrochemistry... a matter of molecules 71.3 Astrochemistry... a matter of moleculesIt is widely recognised by astrophysicists that understanding the chemical processingof interstellar gas clouds is the key to understanding aspects of the physics of starformation (Williams 1998).Since the pioneering papers by Herbst & Klemperer (1973) and Watson (1973),which established the basis of molecular astrochemistry, a great deal of progress hasbeen made in building comprehensive models of the processes and reactions involvedin molecular formation and destruction. Astrochemistry is today a firmly establishedsubject, describing the pathways by which the molecules are formed and destroyed,and in those processes identifying new fundamental chemistry (Fraser et al. 2002b). Inaddition, molecules are of great importance in astronomy because they act as excellentprobes of the physical properties and dynamics which characterise the astronomicalregions in which they are located. Molecules also play an active role in the energybalance of interstellar clouds, affecting the rate of ionisation and the thermal balance,and thus inhibiting or triggering dynamical instabilities. Indeed, through moleculeswe manage to understand how galaxies and stars are formed, how new stars interactwith their environments, and how old stars evolve and die.Molecules are surprisingly found in harsh environments, such as regions sweptby intense UV radiation and energetic cosmic rays fluxes. So far, the inventory ofinterstellar and solar system molecules numbers well over 130 molecular species; theyhave been observed mainly at centimeter- and millimeter-wavelengths where mostmolecules leave their “fingerprint” via characteristic rotational transitions. They aregenerally simple molecules of a few atoms 2 . However, it must be said that almost allof the molecular mass in the Universe is in hydrogen molecules (H 2 ).Interstellar chemistry implies the existence of a great many more species that havenot so far been detected. Many of the detected species are simple molecules familiaron Earth, such as carbon monoxide (CO, the second most abundant molecule, 0.3%by mass), water (H 2 O), ammonia (NH 3 ), methane (CH 4 ), methanol (CH 3 OH), hydrogencyanide (HCN), hydrogen sulfide (H 2 S), or sulfur dioxide (SO 2 ). Moreover, thesimultaneous presence of different ionised species suggests that ion-molecule reactionsare likely to be important in the chemistry. The existence of these ions demonstratesthat a source of ionisation must be present; otherwise, they would rapidly recombinewith electrons to form neutral species. The ionisation source is the flux of cosmicrays, energetic protons, helium nuclei, and electrons, that can penetrate all but thedensest regions of interstellar space. There have also been detections of species thatwere not expected, like the carbon chain molecule HC 11 N. The appearance of theseheavily unsaturated species in a hydrogen-rich environment had not been predicted.2 An updated list of identified molecular species can be found athttp://www.cv.nrao.edu/ awootten/allmols.html

8 Chapter 1. Interstellar dust, gas, and chemistryThere is evidence that even larger molecules exist in the interstellar medium. Thediffuse interstellar bands (DIBs) and the unidentified infrared bands (UIBs) both seemto require the existence of structures that are either large molecules or super smalldust grains (Porceddu 2000, Mulas 2000, Mulas et al. 2003). These structures aresupposed to contain on the order of a hundred of atoms. They are, therefore, likelyto include hydrocarbons, since these molecules are easily capable of reaching such asize.1.3.1 Gas-phase chemistryIt is now well established that the chemistry of simple molecules in gas-phase proceedsmostly by sequences of two-body ion-molecule processes. Under the general ISMconditions, in fact, three-body gas-phase reactions are very unlikely to occur, andthus their contribution can be neglected.As far as diffuse clouds are concerned, UV radiation and cosmic rays easily getthrough and ionise some atoms. Atomic ions then react with H 2 in ion-moleculereactions, and initiate a chemical network that leads to the formation of many ofthe fairly simple molecules observed. However, these simple chemical models do notaccount not only for the observed abundance but also for the mere existence of somesimple species such as CH + . One way to deal with the apparent difficulties of simplemodels is to recognize that surface reactions on dust grains (see next section) are likelyto form hydrogenated species that, in turn, contribute to the network of gas-phasereactions. Another approach to overcome the problems that emerge using simplechemical models is to assume that the interstellar medium is not static, but that itis in continuous dynamical evolution due to very energetic processes occurring withinit, e.g., dissipation of turbulence (Flower & Pineau des Forêts 1998), or MHD shocks(Joulain et al. 1998). In this sense, a high temperature chemistry would explain suchhigh abundances of CH + and the high excitation of H 2 rotational levels. In addition,very small scale (10 AU) and very dense (10 4 cm −3 ) knots were considered as a possibleexplanation of the relative high abundance of HCO + (Cecchi-Pestellini & Dalgarno2000).In molecular clouds starlight is efficiently blocked by dust, present in grains ofvarious sizes and materials, and the gas is irradiated only by cosmic ray particles(mainly MeV protons and electrons). Yet for a rapid chemistry to occur, a thirdbody is needed. But even at the relatively high densities of molecular clouds (10 2 -10 3 cm −3 ), only two-body encounters occur; this means that reactions are usuallyexchange reactions of the type:A + BC −→ AB + Cin which a new molecule AB is formed from the original molecule BC. Such reactionsare much more efficient if one partner is ionised. It is true, though, that direct ex-

1.3. Astrochemistry... a matter of molecules 9change reactions of neutral atoms with H 2 do not occur at very low temperature. SinceH 2 and He are the dominant species, the major initial ions produced are H + 2 and He + .The exothermic reaction H + 2 + H 2 −→ H + 3 + H is rapid and thus provides H + 3 ionsthat are effective proton donors in ion-molecule reactions, creating a variety of newmolecular ions from which new neutral species form (e.g. O + H + 3 −→ OH + + H 2 ).As to He, having a high ionisation potential (24 eV), reactions involving its ionHe + can ionise most neutral species other than H 2 , and dissociate molecular species(e.g. He + + CO −→ He + C + + O). So, the reaction with primary ions produce secondaryions such as C + , N + , O + , N + 2 , O + 2 , HCO, N 2 H + , CH + 3 . Among the numerousneutral species formed, through diverse paths, by subsequent recombination reactions,are CH, CH 4 , NH, NH 2 , NH 3 , NO, CN, until more complex molecule such as methanol(CH 3 OH), and the amminoacid glycine (NH 2 CH 2 COOH).1.3.2 Grain-surface chemistryAlthough many species of molecules detected in the gas-phase of interstellar cloudsappear to be formed via sequences of gas-phase reactions (Herbst & Klemperer 1973,Herbst 2001, Smith et al. 2004) this is not the case for all species in all types ofenvironments.The most important exception (that I shall discuss in detail in one of the sectionsbelow and in Chapter 3) remains the formation of molecular hydrogen, a processthat must be very efficient to form the large amounts of H 2 observed even in diffuseclouds. The radiative association of two hydrogen atoms is a process too rare to beefficient because it involves forbidden roto-vibrational transitions, and gas-phase threebody reactions are too infrequent in the ISM to explain H 2 abundance. Therefore,the widely accepted mechanism for the formation of H 2 in the ISM is that it formsthrough a catalytic reaction between hydrogen atoms on the surface of cosmic dustgrains, where the grains act as a third body in the H + H reaction.The formation of molecular hydrogen is certainly not the only example of surfacechemistry that can occur in interstellar clouds. An atomic hydrogen plays actuallya crucial role, as a reactant, in other processes too; this is because, until totallyconverted to their molecular form, hydrogen atoms are very abundant and their superlight mass allows them to diffuse quickly over a surface if they are only physisorbed;in addition, they are very reactive too. Thus, besides reacting among themselves,hydrogen atoms are thought to react with slower-moving heavier species that stick todust grains (Hasegawa et al. 1992). Most reactions are, as is the recombination ofH 2 , association reaction, where one product is formed and stabilized by a third body;namely, the dust grain. Among the most important processes are those that convertatomic oxygen into water ice:O + H −→ OH,

10 Chapter 1. Interstellar dust, gas, and chemistryOH + H −→ H 2 O,which are believed to be the main production route leading to the large abundance ofwater ice in dense cold regions. Similar mechanisms convert atomic nitrogen into ammonia,and atomic carbon into methane. There is also evidence that carbon monoxide,formed in the gas-phase, can be hydrogenated into formaldehyde (H 2 CO) and evenmethanol (CH 3 OH), even thought some chemical activation energy is involved in theprocess (Hidaka et al. 2004). Besides reactions involving hydrogen atoms, there areother associations reactions thought to be critical, such as the one leading to thespin-forbidden production of CO 2 :CO + O −→ CO 2 ,for which laboratory tests have not yet provided satisfactory results (Roser et al.2001).Also, through FUSE ulraviolet absorption spectra, recently a higher abundancethan expected of molecular nitrogen was detected along the diffuse (A V = 1.5) lineof sight HD124314 (Knauth et al. 2004). The measured abundance of N 2 , despitebeing low for a dense cloud, is still too high with respect to gas-phase diffuse cloudchemistry models; such inconsistency can only be explained either by assuming thatcurrent gas-phase diffuse cloud chemistry models are wrong or by assuming the excessN 2 to be formed via grain-surface reactions.1.4 Molecular hydrogen in space1.4.1 Astrophysical importance of H 2Molecular hydrogen is the simplest of neutral molecules but, in spite of its simplicity,plays a fundamental role in most fields of modern astronomy. In the early universe,the hydrogen molecule was the first molecule to be formed and it played a crucialrole in the collapse of the first cosmological objects because of its high efficiencyas a coolant compared to atomic hydrogen. Molecular hydrogen is by far the mostabundant molecular species in the Universe, able to survive in hostile environmentsand actually detected in a great variety of astronomical objects, including the generaldiffuse interstellar medium, dark clouds, reflection nebulae, H II regions, planetarynebulae, planets, regions of low-mass and high-mass star formation, star atmospheres,supernova remnants, galactic nuclei, and quasars.H 2 is a symmetric molecule, does not have a permanent dipole moment, thereforeits pure rotational transitions are too weak to be detected at radio wavelengths fromground-based telescopes. However, molecular hydrogen gives rise to absorption andemission lines at ultraviolet and infrared wavelengths (Black & Dalgarno 1976, Shull

1.4. Molecular hydrogen in space 11Figure 1.2: Potential energy curves involved in IR and UV transitions for H 2 .Roueff et al. (2000).From& Beckwith 1982, see also Fig. 1.2) that can be detected from space using the modernorbiting observatories, e.g., ISO (infrared domain) and FUSE (ultraviolet domain).Its specific radiative and collisional properties, which we understand reasonably, makeH 2 a diagnostic probe of unique capability because we can construct realistic modelsbased on the response of H 2 to its surroundings (Browning et al. 2003, Cecchi-Pestelliniet al. 2005).Besides, the formation of H 2 on grains in interstellar regions initiates the chemistryof interstellar clouds. As a matter of fact, hydrogen molecules intervene, either in theircharged or neutral form, in virtually all reaction schemes leading to the formation ofmore complex molecules in the ISM. For example, the H + 2 ions produced by cosmicray ionisation of H 2 react with H 2 to create H + 3 ions— detected in the interstellar gasby infrared absorption (Geballe et al. 1999)— which participate in proton transferreactions with neutral atoms and molecules. Thus without formation of H 2 on dustgrains (cf. next section) the molecular component would have been severely limitedin abundance and variety.

12 Chapter 1. Interstellar dust, gas, and chemistry1.4.2 Mechanisms of H 2 formation on dust grainsThere has always been a great deal of interest in the formation mechanisms of H 2 inspace. This is because, besides being such a crucial species, molecular hydrogen needsto be continuously produced in the ISM, owing to ultraviolet photons, cosmic rays,shocks and chemical processes that continuously contribute to its destruction.Gould & Salpeter (1963) first showed that interstellar H 2 formation in the gasphase is not efficient; the simplest gas-phase formation route, the direct radiativeassociation of two hydrogen atoms,H + H −→ H 2 + hν,cannot efficiently occur because the release of the binding energy upon formation of aH 2 molecule via radiative decay is not allowed by selection rules, while its formationthrough the two-step reaction,H + e −→ H − + hνH + H − −→ H 2 + e −occurs efficiently only at high temperatures, as occurs for example in the early universe(Lepp et al. 2002).Therefore, in the cold interstellar medium, Gould & Salpeter (1963) proposed thatinterstellar dust grains can act as catalysts in hydrogen recombination:H + H + dust −→ H 2 + dust, (1.1)with one or both of the hydrogen atoms adsorbed on the surface of a dust grain whenthe reaction takes place. In this case, the dust grain serves to stabilize the newlyformed H 2 molecule by absorbing part or all of the excess energy released.Then, after 1963, a wealth of theories upon understanding the recombination of H 2on dust grain surfaces emerged. Hollenbach & Salpeter (1970) were the first to developa quantum mechanical model to determine the surface mobility and recombinationefficiency of two H atoms on a grain surfaces. One year later, Hollenbach & Salpeter(1971) and Hollenbach et al. (1971) reevaluated the process of molecular hydrogenformation and calculated the efficiency with which two H atoms could recombine ona surface. They considered the grain not as a perfect substrate but as a surface withirregularities and demonstrated that dust grains can successfully catalyse molecularhydrogen production at a rate which is compatible with its observed abundance.Hollenbach et al. (1971) also provided an expression for the steady-state productionrate of molecular hydrogen on grains:R H2 = 1 2 n Hv H n g σγ. (1.2)

1.4. Molecular hydrogen in space 13In this relation of R H2 (cm −3 s −1 ), n H (cm −3 ) is the number density of H atoms,v H = √ 8k B T gas /πm H (cm s −1 ) is their typical speed, n g (cm −3 ) is the number densityof dust grains and σ (cm 2 ) is the average grain cross-section. The parameter γ isthe fraction of H atoms striking the grain that eventually form a molecule, namelyγ = ξη. 0 ≤ ξ ≤ 1 is the sticking coefficient, while 0 ≤ η ≤ 1 is the probabilitythat an adsorbed H atom on the surface recombine with another H atom to form H 2 ,namely, the recombination efficiency.A simplified version of Eq. 1.2 (R H2 = Rn H n) leads to a relation between the rateR H2 , the H 2 photo-dissociation rate, and the total number density of hydrogen nuclei.This relation has often been used so far for estimating the H 2 formation rate alongcertain lines of sight. It takes the formRn H n = βn H2 GS, (1.3)and represents the local balance between formation and photo-dissociation of H 2 (Duley& Williams 1984). In Eq. 1.3 n H , n H2 , and n (= n H + 2n H2 ) are the atomic,molecular and total hydrogen densities (cm −3 ), β (s −1 ) is the photo-dissociation ratein absence of shielding, G is the radiation field value and S is the shielding factorincluding dust extinction and H 2 self-shielding.Jura (1975) calculated, on the basis of an analysis of Copernicus observationsin the local diffuse medium, that the rate coefficient of H 2 formation on grains is10 −17 < R < 3 × 10 −17 (cm 3 s −1 ). Since then, R has been measured in several ISMenvironments. Remarkably, these analysis gave a value of R very similar to that estimatedby Jura (1975) for local diffuse clouds. Recently, Gry et al. (2002) studiedthe formation rate of H 2 along three diffuse lines of sight, using FUSE spectra, whileHabart et al. (2004) estimated R (≃ 10 −16 cm 3 s −1 ) in photon-dominated regions,using SWS-ISO data.There are two main mechanisms of heterogeneous catalysis on a solid: the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) reactions (Zangwill 1984).The ER mechanism ascribes the formation of a molecular species on the surfaceof a catalyst to the direct interaction of an adsorbed radical with an atom hitting itupon arrival from the gas phase. A similar, and not necessarily distinct, mechanismis the hot atom process; here the gas-phase atom lands near the stationary surfaceatom, and is able to move to it along the surface before being completely thermalised.In the diffuse cloud environment, the ER mechanism is not expected to occur becausethe dust grains are bare, the density is low and thus the chance of a H atom hittinganother already adsorbed on the surface is slim.Although the actual mechanism for the reaction is still in some dispute, the mostprobable one is the so-called Langmuir-Hinshelwood mechanism, namely, when molecularformation occurs through a reaction between two atoms (or radicals) that are both

14 Chapter 1. Interstellar dust, gas, and chemistryalready adsorbed on the surface. In order to form molecular hydrogen through the LHmechanism, the following processes must occur: an H atom lands on the surface, exchangesenergy until it becomes thermally accommodated with the surface and stickswith high efficiency, diffuses rapidly over the surface, finds a previously adsorbed Hatom, forms a temporary chemical bond that is stabilized by the release of the excessenergy directly to the grain.Therefore, in order to have the formation of molecular hydrogen, a H atom needsto diffuse on the grain surface, either by tunnelling or thermal hopping, to recombinewith a H atom partner. The mean temperature of dust grains in diffuse clouds rangesbetween 15 and 20 K; thus, if H atoms are to diffuse, weak binding forces (physisorptionforces) must act between the atoms and the surface, or little diffusion wouldoccur otherwise. As far as strong binding sites are concerned, known as chemisorptionsites, these would not lead to diffusive motion and would hinder H 2 formation atvery low temperatures. Chemisorption does not occur at such low temperatures formany systems because there is often an activation energy barrier to the formation ofthe surface-atom bond. Yet, on account of the same reasons, chemisorption is thoughtto favour H 2 formation in regions with higher grain and gas temperatures (10-100 Kand 300-1000 K respectively), as is the case in PDRs (Hollenbach & Tielens 1999,Baouche et al. 2005).1.5 Laboratory experimentsAstronomical data come mainly from on-ground and in-orbit observations which actuallyspan nearly the whole electromagnetic spectrum, namely, from radio (also atmillimetric and sub-millimetric wavelengths) until X-ray and γ-ray observations. Also,a lot was learned from theoretical and computational studies, and from experimentalinvestigations. Each field of research provides us with insights into the nature ofstellar and interstellar phenomena, and the physical and chemical processes involved.Nevertheless, among the most interesting results accomplished in astrophysics, we findthose achieved by combining observational and theoretical data with the outcome oflaboratory work.In relation to the subject of more concern here, astrochemistry, we stress theimportance of the following points:• the identification of new molecules relies on precise laboratory spectra. It is,thus, a challenging task for laboratory groups world wide to provide precisespectral data of molecules which might be of astrophysical importance;• the models based on laboratory experiments provide clues to the ion-moleculereactions in the gaseous phase as well as chemical reactions on grain surfaces;

1.5. Laboratory experiments 15• gas-phase formation routes to many of the observed molecules are still uncertain,so that solid-phase syntheses are of interest in order to reconciliate the predictedand the observed abundances. Low-temperature reactions are thought to occurwithin interstellar ices, upon ice and grain surfaces in the interstellar medium,and on icy surfaces of solar system objects. Ionising radiation, such as cosmicrays and far-UV photons are two possible initiators of such chemistry that canactually be simulated for the most part in laboratory.With special regards to grain-surface chemistry leading to formation of H 2 , thereexist a wealth of theoretical works but not many experimental investigations. Due tointrinsic experimental difficulties, only a few laboratory attempts have been performedto measure the efficiency of the catalytic process. Of course, the best understandingof the problem necessarily needs the results of experimental investigations that simulatethe occurrence of the process of H 2 recombination in conditions close to thoseencountered in interstellar clouds and that provide values of those parameters formore realistic theoretical descriptions. It is not rare that theoretical and computationalstudies have to estimate or consider unknown some important parameters (e.g.,activation energies) because of lack of experimental data.Besides, surface chemistry is generally far more poorly understood than gas-phasechemistry, and in the case of interstellar dust grains, there are additional problemsbecause we only have an insufficient knowledge of the physical nature of the surface.Here are some problems that still concern experimental astrochemists:i) detailed mechanism for the formation of molecules (Langmuir-Hinshelwood vs Eley-Rideal) in several ISM conditions;ii) the dependence of the rate of surface reaction on grain size, and grain morphology;iii) detailed mechanism for the desorption from grain surfaces and excitation statusof desorbing molecules.In recent years, a number of useful experiments have been undertaken on thesurface formation of molecular hydrogen and other species at low temperatures onrealistic analogues of interstellar grains. I shall review the results of most of theseexperiments in Chapter 3. For intrinsic experimental problems, all experiments havebeen performed on cold surfaces rather than small particles. The experiments on H 2formation have been of two types: 1) those aimed at determinig the internal andtranslational energy of the molecules as they leave the surface (Perry et al. 2002)and 2) those designed to understand the mechanisms and measure the rate of therecombination process (Pirronello et al. 1997a,b, 1999, Manicò et al. 2001, Roseret al. 2002, 2003, Hornekær et al. 2003). The major concern in this thesis will beexperiments of type 2).These experiments were most often accomplished by the technique of TemperatureProgrammed Desorption, or TPD for short, in which the H atoms are deposited with as

16 Chapter 1. Interstellar dust, gas, and chemistrysmall a deposition rate as possible on surface kept at a specific temperature between 5and 30 K, and then the temperature is raised slowly, leading to both diffusive reactionand desorption of the product.In the case of studies upon formation of H 2 O or more complex and refractorycarbon-bearing molecules, the TDP technique cannot be applied, thus it is often assisted(or replaced) by another kind of analytical technique called FT-RAIRS, that isshort for Fourier Transform Reflection Absorption infrared (IR) Spectroscopy. Indeed,IR spectroscopy was among the first techniques to be applied to the direct characterizationof adsorbates. The early studies, though, employed transmission infraredspectroscopy, in which the IR beam passes through a suitably prepared sample andthe change in its absorption spectrum is monitored. The development of FT-RAIRSstemmed from the definite advantage of enabling one to investigate adsorption ofgaseous species on a bulk reflective metal without the complication of introducingtransparent and very thin substrates. I shall discuss this topic in detail in Chapter 4.In brief, FT-RAIRS consists of reflecting an infrared beam off a metal surface onwhich an adsorbate was previously deposited, and then looking at the loss of intensityof the reflected light at frequencies which correspond to the vibrational modes of theadsorbed species. As it turned out, at high angles of incidence (near 80 ◦ ) a very highsensitivity can be obtained in a reflection-absorption experiment.1.6 This thesisCentral to this thesis is the chemistry occurring on dust grain surfaces leading to theformation of molecules in the ISM, and, in particular, the laboratory simulation offormation mechanisms and formation rates. Surface chemistry plays a crucial role inthe ISM because it produces key species that are not formed in gas-phase reactionsat an efficient rate. Among them, molecular hydrogen is by far the most important.In this work (Chapter 3), I shall address the experimental investigation of H 2formation on diverse samples of amorphous silicates. The experimental work wasconducted in the Physics Department laboratories at Syracuse University, New York,as part of the most successful programme of experiments so far to study the processesinvolved in the formation of molecular hydrogen on a variety of dust analoguematerials, also including poly-crystalline olivine, amorphous carbon, and ices. Theexperiments were carried out through mass spectrometry and TPD techniques andunder conditions that come as close as technically feasible to the ones in the most relevantISM environments, namely, under ultra high vacuum pressures (low 10 −10 torr)and at surface temperatures between 6 and 30 K. Experimental studies of H 2 formationon amorphous olivines are of major concern in grain-surface chemistry becauseamorphous silicates are believed, together with carbonaceous materials, to be themost realistic analogues of bare cosmic dust surfaces in diffuse clouds. In my doctor-

1.6. This thesis 17ate work I carried out numerous experiments on a set of several samples of amorphousolivines of the type (Mg x ,Fe 1−x ) 2 SiO 4 , namely, samples made up of diverse amountsof Mg and Fe.Besides, in Chapter 4, I shall address the project and the construction of a FT-RAIRS facility that is to integrate the existent research apparatus in the laboratoryat Syracuse University. I shall first discuss the FT-IR spectroscopy, then I shall focuson a particular technique used in surface science called RAIRS. Its physical principlewill be discussed as well. Finally, I shall describe “piece by piece”, the design and theconstruction of the FT-RAIRS arrangement.

18 Chapter 1. Interstellar dust, gas, and chemistry

Chapter 2The Laboratory ofSurface Science at SUThe laboratory of Surface Science and Astrophysics is located in the subbasement(two floors underground!) of the Physics Department at Syracuse University (SU),Syracuse, NY. During my doctorate work, as a visiting PhD student, I carried outall the experimental work in this laboratory. Here I had the possibility to become familiarwith practical experimental problems, experimental techniques, and laboratoryequipment dedicated to the study of astrochemical grain-surface reactions.The research group is headed by Prof. Gianfranco Vidali (SU), and Prof. ValerioPirronello (University of Catania, Italy). The group is also composed by formergraduate student at SU Joe Roser (he got his PhD by now), Ling Li (graduate studentat SU), and Dr. Giulio Manicò (University of Catania).In this chapter, I shall describe the laboratory equipment and the experimentalmethods used in the investigation of grain-surface reactions of astrophysical interest.2.1 Overview of laboratory equipmentA schematic top view of the experimental apparatus used at Syracuse University inthe most recent series of measurements is show in Fig. 2.1. The apparatus consistsof a Ultra-High Vacuum (UHV) stainless steel chamber (or, in brief, named mainchamber) evacuated by a cryopump, a turbo molecular pump, and an ion pump(operating pressure is in the low 10 −10 torr range). The main chamber houses asample holder and a rotatable quadrupole mass spectrometer that is mounted at thebottom flange. The sample holder is placed in the center of the main chamber andis mounted onto a liquid helium continuous flow cryostat hanging from the upperrotatable flange. By throttling the flow of liquid helium the sample can be cooledto 4.5 K. Two triple differentially pumped atomic/molecular beam lines are aimed atthe surface of the sample. Each has a radio-frequency cavity in which the molecular

20Chapter 2. The Laboratory ofSurface Science at SUFigure 2.1: Schematic of the apparatus (top view). S1 and S2 denote H 2 and D 2 RFdissociation sources; CH1, and CH2 are the mechanical choppers along the beamlines; T1,T2, and T3 are turbo molecular pumps; Ti is a titanium sublimation pump; and C iscryopump. LV is a leak valve for introducing water vapor into the system through a capillary,and VP is a MgF 2 viewport. Adapted from Roser et al. (2002).species (hydrogen or deuterium in this work) is dissociated, and then injected intothe line. Dissociation rates are typically in the 75 to 90% range, and are constantthroughout a run. The reason for using two beamlines and two isotopes is dictated bythe fact that, in preliminary runs using only one line, it became evident that the signalof H 2 (or D 2 ) formation was hidden in the background given by the undissociatedfraction of molecules coming directly from the beam source. The possibility of usingtwo atomic/molecular beam lines is undoubtedly one of the most important featuresof this equipment.Also, a triply differentially pumped time-of-flight (TOF) line extends from oneof the sides of the UHV chamber. Although it was used in the past to measure thevelocity distribution of particles leaving the sample, the TOF appendix was not usedin this work so I shall not describe it any further.In the next sections I shall discuss more accurately the main parts of the apparatus,such as the beamlines, the main chamber, and the data acquisition system.However, many more details concerning the equipment can be found in Roser (2004,PhD Thesis).

2.1. Overview of laboratory equipment 21Figure 2.2: RF resonator and Pyrex source. Adapted from Slevin & Stirling (1981).2.1.1 BeamlinesThe two beamlines form angle of 38 ◦ and converge on an ideal point lying onto thecentral axis of the main chamber. Each atomic/molecular beam line has three differentiallypumped stages. The deuterium line (D line) is essentially the replica of theH line except for some minor construction details.The first stage houses a water-cooled Pyrex source that is surrounded by a coaxialRF cavity for dissociating molecular gases such as hydrogen and oxygen. Watercooling proves to be essential, not only to reduce the discharge temperature, but alsoto inhibit recombination of hydrogen atoms on the walls of the discharge tube (Wood& Wise 1962). Radio-frequency power from an ENI 300W RF power supply is fed toboth the RF cavities via a power splitter through a custom-made impedance matchingnetwork.During the time spent in the laboratory at SU, I also redesigned and rebuilt one ofthe RF discharge sources according to the model proposed by Slevin & Stirling (1981),see Fig. 2.2, and with technical construction specification reported by MacAlpine &Schildknecht (1959), in order to optimise the new resonator to the feeding RF power at13.56 MHz. The cavity consists of a copper cylinder (ID=6 inches, length B=8 inches),surrounding the discharge tube, and a 26-turn coil (wire type B&S gauge #12) ofconstant pitch, axial length b=5.2 inches and mean diameter of turns d=3.3 inches.Power is coupled to the resonator at the fifth turn from the grounded end of the coilto optimise the coarse impedance matching. The right position of the feeding tapwas determined after repeated tests with the tap position varied in order to find thelocation correspondent to the minimum reflected power. Since there was significantpower loss due to the large volume of water flowing within the old Pyrex source, Ialso redesigned a new discharge tube with a smaller water jacket.The fraction of gas dissociated in the beamline can be measured by rotating the

22Chapter 2. The Laboratory ofSurface Science at SUmain chamber QMS in front of the beamline. The dissociation fraction is given byR dissoc = I RFoff − I RFonI RFoff(2.1)where I is the intensity of the diatomic species with either the RF dissociation on oroff. Typically measured dissociation fractions are beyond 90% for D 2 or H 2 , ∼ 50%for O 2 , and only 5 - 10% for N 2 , due to the greater strength of the nitrogen-nitrogentriple bond.The RF discharge source is mounted on a positioning frame and is connected tothe first stage chamber, evacuated by a diffusion pump, via a flexible coupling. Tocool the hydrogen atoms from the dissociation region, the nozzle is terminated witha short aluminum channel that can be cooled to ∼ 200 K by a copper braid attachedto a liquid nitrogen reservoir. Typical pressures within the chamber of the first stageare between 10 −4 and 10 −5 torr.In the second stage chamber, this too pumped down by a diffusion pump, a 50 %duty cycle mechanical chopper is used to chop the atomic beam on its way from thesource to the sample. Reference pulses from the chopper are obtained from a phototransistor which detects a chopped light beam. The light is emitted by an LED, whichis mounted across the chopper blade on the other side of the chopper mounting rack.There are two main advantages for chopping the beam: we can a) accurately measurethe intensity of the beam flux via in-phase detection, and b) reduce the beam fluxin experiments which require as low a flux as possible. The second stages are alsoequipped with a simple flag shutter. This flag valve enables us to modulate the beam(let it through or block it) very quickly (within 1 second).The third stage is a buffer stage with an operating pressure in the high 10 −9 - low10 −8 torr range, pumped down by a turbo molecular pump.Collimators of 2-3 mm diameter separate each stage from the adjacent one. Estimatedatomic fluxes are as low as 10 12 atoms cm −2 s −1 , while the beams are angularlyresolved to ∼10 −6 steraradian. The two atomic/molecular beams enter the main UHVchamber via two gate valves. Besides, the lines are accurately laser-aligned to aim atthe same spot on the sample.The two beamlines are always under vacuum conditions even when the beams arenot in operation. In fact, the beamlines are isolated from the diffusion pumping linesby gate valves when the diffusion pumps and their mechanical backing pumps are shutoff. Thus the beamlines are continually pumped by the turbo-molecular pump in thethird stages, backed by a mechanical rotary pump, when the lines are not in use.2.1.2 Main chamberThe main UHV chamber is a stainless steel cylinder, 10 inch diameter. It has avolume of ∼ 28 liters, and is evacuated by a turbo molecular pump, an ion pump,

2.1. Overview of laboratory equipment 23a cryopump, and, occasionally used during the experiments, a titanium sublimationpump. In order to reduce the background pressure of light gases, the turbopump isalso backed by a cryo-baffled small diffusion pump which is turned on only duringexperimental runs. The operating pressure in the main chamber is in the 10 −10 torrrange. The main chamber houses the detector, a rotatable differentially pumpedquadrupole mass spectrometer (QMS), and the sample holder.The sample holder was designed to have the sample lying on the central axisof the main chamber at the same height of the beamlines, namely, where the twoentering beams converge. Also, the sample is mounted on a UHV compatible coldfinger bolted onto the upper rotatable flange. The sample temperature is measuredby an iron-gold/chrome thermocouple and a calibrated silicon diode placed in contactwith the back of the sample.The QMS (a Balzers model QMS311) is placed inside an enclosure mounted onthe bottom rotary flange of the chamber and can be rotated by 190 ◦ around thesample. This detector is used for measuring the intensity of the incoming beamsand the products of reaction evolving from the surface of the sample. To reduce thebackground noise, the QMS detector enclosure is differentially pumped by a turbomolecular pump. The bottom rotary platform is sealed with three Teflon rings thatare, in turn, differentially pumped. The outermost cavity is pumped by a rotary vanepump, while the inner cavity is pumped by a small ion pump. The double differentialpumping allows the detector to be rotated around the sample without spoiling theUHV pressure in the main chamber.Prior to each experimental run, the sample is heated to ∼ 400 K for driving impuritiesoff its surface by using a constantan heater (R ∼ 4 Ω) located in a ceramic boxbehind the sample. After cooling, the temperature of the sample can be adjusted byvarying the liquid helium flow to the cryostat and by adjusting the current throughthe heater (currents in the range 0-2 A). In this way, temperatures can be set andmaintained between 5 and 30 K. The source for cooling the sample is provided bypassing liquid helium vapor through a commercial continuous flow cryostat (cold finger).There is an indium foil between the sample and the sample holder to providea better thermal contact between the sample and the cryostat. Furthermore, the lowtemperature stage is surrounded by a radiation shield thermally anchored to the outerstage of the cryostat and has a small square window ( 1 cm 2 ) in the front to let thebeams in and impinge the sample. The shield has no thermal contact with the sampleholder. The sample is 10 mm in diameter and thus larger than the 3mm-diameterbeams. This arrangement prevents the sticking of particles coming from the beams onother parts of the sample holder or the shield. All the parts that are routinely broughtto the lowest temperature are made of oxygen-free high-conductivity (OFHC) copperfor good thermal conductance and cleanliness.

24Chapter 2. The Laboratory ofSurface Science at SU2.1.3 Data acquisitionA personal computer manages data acquisition during an experiment. The silicondiode mounted behind the sample has its voltage read by a control unit that outputsa voltage proportional to the diode temperature. The thermocouple mounted behindthe sample has its voltage, which is approximately a linear function of the sampletemperature, read out and amplified by a digital multimeter. These two temperaturesignals can be simultaneously recorded as a function of time by a National Instrumentsdata acquisition (DAQ) card operated by a graphical user interface coded in C thatwas specially designed for this purpose. The output of the main chamber QMS isprocessed by an Advanced Research Corporation model Combo-100 preamplifier witha maximum pulse counting rate of 50 MHz which far exceeds the count rates generatedduring desorption experiments performed in our laboratory. This preamplifier hastwo output modes: an analog mode in which a voltage proportional to the detectorresponse can be output and recorded by the DAQ card and a pulse counting modein which ion detections by the QMS are output as TTL pulses. These pulses arerecorded by a multichannel scalar (MCS) card operated by proprietary software. TheMCS and DAQ cards are installed on the same computer and can be triggered torecord data simultaneously, namely, desorption rate and sample temperature.2.2 Experimental methods2.2.1 Theoretical considerationsDesorption process and kineticsAn adsorbed species present on a surface at low temperatures may remain almostindefinitely in that state. As the temperature of the substrate is increased, however,there will come a point at which the thermal energy of the adsorbed species is suchthat one of several processes may occur :1. a molecular species may decompose to yield either gas phase products or othersurface species.2. an atomic adsorbate may react with the substrate to yield a specific surfacecompound, or diffuse into the bulk of the underlying solid.3. the species may desorb from the surface and return into the gas phase.The last of these options is the desorption process. Desorption is the rupture ofthe adsorption bond and the resulting removal of adsorbed particles from the surface.It can be accomplished in different ways. Thermal desorption (TD), electron impactdesorption (EID) and electron-stimulated desorption (ESD) have been widely used inthe measurements of desorption processes for defining adsorbate states and calibratingtheir populations.

2.2. Experimental methods 25In our experiments, Thermal Programmed Desorption (TPD, see below) has beenused because it can be easily implemented with our apparatus set-up in order to favourthe desorption process. By carefully controlling the temperature ramp, the timedependentthermal desorption process can be converted to a temperature-dependentprocess which can give additional information on properties of the adsorbate (e.g, thebinding energies) and on desorption kinetics.The starting point for analysis of thermal desorptions is the Polanyi-Wigner equation.This expression gives the rate at which molecules desorb:−dN(t)/dt = k m n(t) m , (2.2)where n(t) is the density of atoms/molecules on the surface at time t, k m is therate constant for the desorption process, and m is the order of the desorption process.The order of desorption can usually be predicted because we are concerned with anelementary step of a “reaction”, specifically:I. Atomic or Simple Molecular DesorptionA (ads)−→ A (gas)M (ads)−→ M (gas)will usually be a first order process ( i.e., m = 1 ). Examples includeCu / D 2(ads) −→ Cu (s) + D 2(gas)desorption of D 2 molecules from a Cu surface;II. Recombinative Molecular Desorption2 A (ads)−→ A 2(gas)will usually be a second order process ( i.e., m = 2 ). Examples includeNi / H (ads)−→ Ni (s) + H 2(gas)desorption of H atoms as H 2 from a Ni surface.The rate constant for the desorption process may be expressed in an Arrheniusform,k m = k (m) exp( ) −Ed, (2.3)k B Twhere E d is the activation energy for desorption, and k (m) is a constant. Thislatter term can also be considered to be the “attempt frequency”, ν, at overcoming thebarrier to desorption. In fact, in the particular case of simple molecular adsorption,the k (m) factor may also be equated with the frequency of vibration of the bondbetween the molecule and substrate; this is because every time this bond is stretchedduring the course of a vibrational cycle can be considered an attempt to break thebond and hence an attempt at desorption.

26Chapter 2. The Laboratory ofSurface Science at SUFor m = 0, desorption is independent of coverage, which is the case appropriatefor desorption from several layers, since the desorption yield doesn’t depend, in firstapproximation, on the coverage. A typical signature is the presence of a commonleading edge for different coverages. The case of m = 1 corresponds to first order desorption,and the molecules already formed on the surface leave during the desorptionindependently of each other; a distinguishing trait is an asymmetrical desorption peakwith the same peak position as a function of coverage. The case of m = 2 correspondsto second order desorption, in which the reaction rate is proportional to the product ofthe concentrations of the reactants. In this case, a symmetrical desorption peak shiftstowards lower temperatures as the coverage increases. See Fig. 2.3 for a comparisonamong the three cases.0 th 1 st 2 ndDesorption rateTimeFigure 2.3: Representative thermal desorption curves with varying initial coverages andwith parameters k (m) and E d held constant.If each molecule is already present on the surface at the beginning of the TPD (them = 1 case discussed above) and if it desorbs independently of the others, then E d hasthe meaning of the energy that it takes to bring a molecule from the adsorption energywell into the gas phase. If, at the beginning of the TPD, the atoms recombinationreaction has not yet occurred (m = 2), then E d is the rate limiting activation energybarrier for the process of setting the atoms in motion, recombining into a moleculeand then desorbing.Typically, it is assumed that E d does not depend on the coverage. In our casethis is justified because we strive to work at low coverage, a small fraction of onelayer, in order to approximate astrophysical conditions where the number of hydrogenatoms simultaneously present on a grain is small. Furthermore, for complex, poroussurfaces such as the surface of amorphous silicates, there is probably a distribution ofdesorption energies E d .

2.2. Experimental methods 27Residence times of adsorbatesOne property of an adsorbed molecule that is intimately related to the desorptionkinetics is the surface residence time. This is the average time that a molecule willspend on the surface under a given set of conditions (in particular, for a specifiedsurface temperature) before it desorbs into the gas phase.For a first order process such as the desorption step of a molecularly adsorbedspecies:M (ads)−→ M (gas)the average time (τ) prior to the process occurring is given by τ = 1/k 1 where k 1 isthe first order rate constant.Thus, assuming a first order kinetics, if we substitute for k 1 its expression asdefined in Eq. 2.3, we get the following expression for the surface residence time( )Edτ = τ 0 exp , (2.4)k B Twhere the constant τ 0 = 1/ν corresponds to the period of vibration of the bondbetween the adsorbed molecule and substrate and is frequently taken to be about10 −12 s.TPD techniqueThere are a range of techniques for studying surface reactions and atomic/molecularadsorption on surfaces which utilise temperature-programming to discriminate betweenprocesses with different activation parameters. Of these, the one utilised in ourlaboratory studies is the thermal programmed desorption.Experimentally, the TPD technique is very simple, and it involves two basic steps:1. Adsorption of one or more molecular species onto the sample surface at lowtemperature;2. Heating of the sample in a controlled manner (preferably so as to give a lineartemperature ramp) whilst monitoring the evolution of species from the surface backinto the gas phase.In modern implementations of the technique the detector of choice is a small,quadrupole mass spectrometer and the whole process is carried out under computercontrol with quasi-simultaneous monitoring of a large number of possible products.The data obtained from such an experiment consists of the intensity variation of eachrecorded mass fragment as a function of time/temperature.Fig. 2.4 shows data from a TPD experiment following adsorption of D 2 onto apolished Cu surface at 12 K. In relation to TPD spectra, the following points areworth noting :

28Chapter 2. The Laboratory ofSurface Science at SU8000Desorption Rate dN/dT @ Mass 4600040002000010 15 20 25 30 35Surface Temperature (K)Figure 2.4: TPD trace from polished Cu following 2 minutes D 2 adsorption at 12 K.• The area under a peak is proportional to the amount originally adsorbed, i.e.,proportional to the surface coverage;• The kinetics of desorption (obtained from the peak profile and the coveragedependence of the desorption characteristics) gives information on the state ofaggregation of the adsorbed species, molecular vs dissociative for example;• The position of the peak (the peak temperature, or T peak ) is related to theenthalpy of adsorption, i.e., to the strength of binding to the surface.One implication of the last point is that if there is more than one binding state fora molecule on a surface (and these have significantly different adsorption enthalpies)then this will give rise to multiple peaks in the TPD spectrum, with the more weaklybound species desorbing at a lower temperature.In the experimental studies of the present work, TPD experiments give us twokinds of information. First, by measuring the number of reactants sent onto thesurface and the number of molecules detected (which is proportional to the area ofthe TPD curve), the efficiency of the recombination reaction can be inferred. If thereis a substantial number of molecules formed during irradiation, then this contributionhas to be added to the total. Second, information on the kinetics of the reactioncan be inferred by studying the desorption pattern. Although in principle one coulddetermine the order of the desorption kinetics and the parameters in the Polanyi-Wigner equation by looking at the shape of the peak (being asymmetrical for m=1and symmetrical for m=2) and the temperature when the maximum occurs, in practicethis is not a reliable method because the peak can be distorted by technical (finitepumping speed) and physical (range of E d ) reasons. Thus, one performs TPDs bychanging either the heating rate or the exposure time (Menzel 1975). Then, one usesEq. 2.2 to determine the order of the desorption and the other parameters. In ourcase, changing the heating rate is not feasible (the heating rate would have to be

2.2. Experimental methods 29changed by orders of magnitude to extract meaningful information); therefore, westudied the kinetics of desorption by changing the exposure, but keeping the coveragewell below the completion of one layer.2.2.2 Importance of the two-beamline configurationThe use of two beamlines is essential in experiments of molecular hydrogen formation.Although the efficiency of dissociation in the sources is up to 90%, there is still anunwanted undissociated fraction of molecules that gets deposited on the surface. Hydrogenmolecules are also the major residual gas in a well-baked and clean stainlesssteel UHV chamber, so the high H 2 background allows the molecules to stick on thesurface of the sample during measurements. Finally, during a thermal programmeddesorption, H 2 might be desorbed from other parts of the sample holder as the temperatureis ramped up. It was then decided to use a beam of H and one of D in orderto analyse the formation of HD instead, minimising many technical difficulties. HDcan actually form only on the sample surface and this results in a huge increase of thethe signal to noise ratio.The use of two beams with different isotopes indisputably proved to be an irreplaceabletool in separating reactions effects occurring on the surface from the contributionsof the background gas. Indeed, our group was able to achieve very importantfindings upon the formation of molecular hydrogen on dust grain analogues that hadnot been possible previously.2.2.3 Beam intensitiesIn order to determine the amount of gas exposed to sample, the intensity of thedeuterium and hydrogen beamlines are measured before any thermal desorption measurementsare carried out. Shown in Fig. 2.5 is a measurement of the D 2 line beamintensity with the RF dissociation both on and off. With A D2 ,rfoff and A D2 ,rfon as thearea of the pulses above the background level, the beam intensities of D 2 are given byf D2I D2 ,rfoff = A D2 ,rfoff(2.5)n passesf D2I D2 ,rfon = A D2 ,rfon(2.6)n passeswhere f D2 is the frequency at which pulses of gas pass through the deuterium linechopper and n passes is the number of passes of the multichannel scalar summed toproduce the measured graph of beam intensity versus time (the graphs in Fig. 2.5have n passes = 1000, for example). The beam intensity of deuterium atoms is givenbyI D = 2R D2 ,dissoc I D2 ,rfoff (2.7)

30Chapter 2. The Laboratory ofSurface Science at SUwhere R D2 ,dissoc is the dissociation rate given by Eq. 2.1. A similar calculation for thehydrogen line is used to calculate I H .3000QMS signal @ mass 4 (counts per 40 s)250020001500100050000 4 8 12 16 20 24Elapsed time (ms)Figure 2.5: Comparison of D 2 beam intensity with the RF off (filled circles) to the D 2beam intensity with the RF on (open circles). Commonly, as in this case, the RF power is100W divided between the two atomic sources.Determining the atomic beam intensity of each beamline in terms of measurementsmade on the molecular gases allows calculations of the recombination efficiency to bemade without determining the relative ionisability of D or H atoms in the detectorversus their molecular counterparts. A trace amount of molecular HD is detectedfrom the deuterium beamline, although the ratio of HD molecules to D atoms in thebeam with the RF power on is typically less than 1% and this trace amount of HDcan be subtracted from the desorption yield of HD.2.2.4 Adsorption and TPD proceduresAfter cooling to ∼ 4.5 K, the sample is brought to a fixed temperature (adsorptiontemperature) and is exposed for a given amount of time (exposure) to the H and Datomic beams. The beams are introduced into the main chamber and aimed at thesample target. The exposure is modulated by a flag valve present in each beamline.During this time, the atoms or radicals that got trapped on the surface might react,either because their mobility is high, or because they land on atoms or radicals thatwere already present on the surface and react (i.e., recombination via the Eley-Ridealmechanism). Then newly formed molecule can either use the energy gained in makingthe bond to desorb or it might remain on the surface.As the sample is being exposed to H and D, it is positioned to face directly inbetweenthe beamlines and the QMS detector is positioned between the incomingbeams, in front of the sample window, to detect the flux emerging normal to the

2.2. Experimental methods 31Figure 2.6: Main chamber HD background level measured during the adsorption phaseof the experiment with a sample temperature of 10 K. The beam was let in at time = 20seconds.sample surface. The detector is used to record the background level of HD in themain chamber for 20-30 seconds before measuring the increase in the backgroundlevel once the atomic beams are admitted into the main chamber. This stage of anexperiment is referred to as the adsorption phase and this MCS measurement saved onthe PC computer is referred to as the adsorption file. An example of the backgroundrise measured when admitting the H and D beams into the main chamber on a sampleof amorphous silicate at 10 K is shown in Fig. 2.6. The background rise was alwaysobserved to be constant during the whole irradiation period, so it was only necessaryto record the HD background rise for the initial portion of the beam exposure.Once the exposure of the sample with atomic H and D (H+D for short) is completed,a TPD measurement is carried out by providing the sample with a heat pulse.This procedure essentially shortens the time that the individual reactants would havetaken to move about the surface and react (i.e., recombination via the Langmuir-Hinshelwood mechanism).During this phase the main chamber QMS signal is measured for a preset fixedmass (e.g., mass 3 for HD) by the MCS card at the same time as the silicon diodeand thermocouple outputs are measured by the DAQ card. The QMS signal andthe temperature sensor outputs are simultaneously recorded for 10 seconds with thebeamlines closed and with the sample still at the temperature used for the adsorptionto establish the background level. The measurements continue as the flow of liquidhelium is shut off (this is how the heat pulse is given) and the sample temperature

32Chapter 2. The Laboratory ofSurface Science at SU25Sample temperature (Kelvin)211713950 10 20 30 40 50 60 70 80 90Elapsed time (s)Figure 2.7: Sample temperature ramp as a function of elapsed time diring a TPD experiment.The liquid helium flow was shut off at an elapsed time of 10 seconds.is allowed to rise. Stopping the liquid helium flow to the cryostat gives a highlyreproducible sample temperature ramp for a given adsorption temperature, althoughthe slope of the temperature ramp will depend upon the amount of heat supplied bythe sample heater to maintain the starting sample temperature. The temperatureramp can be fit with excellent results with the empirically determined functionT (t) = a t + bc + t + d t 1 3 (2.8)in the rapidly rising portion of the temperature ramp. An example of a temperatureramp starting from 5.2 K is shown in Fig. 2.7.The desorption and temperature files recorded during the thermal desorption measurementprovide information about the desorption rate dN/dt of HD from the surfaceand the temperature T (t) of the sample versus time. The desorption traces are convertedfrom dN/dt to dN/dT by subtracting the background level from the desorptionfile, and then by dividing the dN/dt points by the corresponding values of dT/dt basedon a curve fit to the temperature file.2.2.5 Desorption yield and recombination efficiency calculationsAn example of a thermal desorption spectrum recorded by the MCS card is shown inFig. 2.8. The area of the desorption peak can be computed in exactly the same wayas the area of one of the peaks acquired when measuring the beam intensity. Theaverage background level can be determined from channel 0 to ∼ 100.

2.2. Experimental methods 33120Counts per channel90603000 300 600 900 1200Channel number (100 ms dwell time)Figure 2.8: Thermal desorption measuring mass 3 (for HD) after 2 minutes H and Dexposure of an amorphous silicate sample at ∼ 5 K.However, the true desorption yield of gas from the sample surface in units of“counts” is not given by the desorption peak area directly because certain correctionsapply. The first problem is that the small size of the detector aperture only interceptsa small fraction of the gas desorbing from the sample surface. Previous researchershave calculated that dividing the peak area by a factor of C a = 3.5844×10 −3 providesthis correction for our experimental setup.The second problem is that the detection efficiency of the QMS is inversely proportionalto the velocity of the gas desorbing from the sample (the faster gas particlesspend less time in the QMS ioniser and have a smaller chance to be ionised and thendetected). So approximating this problem to the case of an ideal gas, we can assumethe velocity of the desorbing gas particles is proportional to the square root of thetemperature. Since the detection efficiency is inversely proportional to the velocity,we have1Area ∝ Y ieldv(T peak ) ∝ Y ield 1√ (2.9)TpeakNow, if we compare the desorption yield to the beam intensity, the latter correspondingto particles emerging at T ∼ 300 K, we can multiply the desorption peakarea by the QMS detection efficiency at the temperature of the gas emerging from thebeamline. Since the constant of proportionality cancels out in this case, we haveY ield = Area C t , with C t =√Tpeak300K(2.10)A final correction that needs to be applied to the desorption peak area to getthe desorption yield is due to an empirical observation that the desorption peak areadepends upon the angle at which QMS is placed with respect to the normal of the

34Chapter 2. The Laboratory ofSurface Science at SUsample. In fact, even with the detector rotated 90 ◦ from the sample normal an appreciableamount desorption peak is still observed, due to the rise in the backgroundpressure of the desorbing gas in the main chamber. This correction factor too for thebackground correction was calculated previously and it takes the value of C bg = 0.35.The final equation for determining the desorption yield from the peak area is thengiven byY ield = Area C bgC a√Tpeak300K(2.11)As far as the recombination efficiency of HD is concerned, the logical method tocalculate it is to compare the desorption yield to the amount of gas exposed to thesample. Thus, with the two beamlines, one sending D atoms and the other sending Hatoms into the main chamber, we can look for the desorption yield of HD moleculeswithout any confusion, since no other molecule likely to form on the surface has mass3 as HD. What we do is measure the beam intensities I H and I D of H and D atomsusing Eq. 2.7, the esposure time t exp of the beams to the surface, and the desorptionyield of HD molecules from the surface with Eq. 2.11. The recombination efficiencyis then simply:Sγ = Y ield HDN HD(2.12)where N HD is the number of HD molecules that would form in a hypotheticalexperiment in which N H = I H t exp hydrogen atoms and N D = I D t exp deuterium atomsstick to the sample with a sticking coefficient S = 1 and recombine perfectly randomlywith recombination coefficient γ = 1. Using the probability theory we can calculateN HD , see appendix of Roser (2004)’s thesis, to beN HD =N HN DN H + N D(2.13)If a small amount of HD emerging from the beamlines is detected, the total amountof HD exposed to the sample during the exposure time can be subtracted from thedesorption yield assuming S HD = 1, although this amount of HD is usually neglectableand typically we do not measure S HD directly in our experiments.2.2.6 Typical protocol of measurements• The sample is heated to ∼ 400 K for cleaning purposes• Beamlines are turned on and the beam intesities and dissociation fractions aremeasured with the QMS. The beamvalves are then shut off;

2.2. Experimental methods 35• Connection of the liquid He line to the cryostat and then initial cooling downof the sample to ∼ 4.5 K;• The sample is reheated to 250 K to drive off impurities condensed on the samplesurface during initial cooldown, the beamvalves and the liquid helium flow arereopened. Cooling down again;• Set of the adsorption temperature by modulating the heater current;• Series of H+D and D-only irradiations to the sample;• QMS set on mass 3, TPD run(s). After each TPD experiment the temperatureis let go up to ∼ 50 K to be sure that the adsorbates desorb completely (HDdesorption mostly occurs between 15 and 35 K).

36Chapter 2. The Laboratory ofSurface Science at SU

Chapter 3Molecular Hydrogen Formation onAmorphous OlivineThe study of the formation of molecular hydrogen in the ISM has a special significancein astrophysics; molecular hydrogen, either neutral or charged, enters most of thereaction schemes of the formation of molecules in the ISM, and it has an importantrole in cooling a gravitationally collapsing gas cloud.As discussed in Chapter 1, the abundance of molecular hydrogen in the interstellarmedium can be explained by hydrogen recombination reactions taking place on thesurface of interstellar dust grains.Despite theoretical progress upon H 2 formation in space, and the past experimentscarried out by our group, there remained important unresolved issues. The previousexperiments actually demonstrated, on the ground of strong evidence, that the processdoes take place on astrophysically relevant dust grain analogues. However, twomain questions still remain unanswered: a) does the catalytic efficiency of amorphousgrains account for observations of actual H 2 abundance in various interstellar mediumenvironments? and b) what are the mechanisms that allow H atoms to migrate onthe grain surface?The experiments described here were planned to investigate for the first timehydrogen recombination on a series of amorphous silicate surfaces. In fact, amorphoussilicates represent one of the primary dust grain analogues that chemically andmorphologically best replicate interstellar dust grains in diffuse cloud environments,where the formation mechanism of molecular hydrogen has to be efficient to balanceits destruction by ultraviolet light.The experimental results presented in this work answer, at least in part, the twoquestions above by showing that the catalytic efficiency of amorphous silicates allowsH 2 to be formed in larger quantities and over a wider temperature range than previouslyknown, and close to the one measured on amorphous carbon surface, which isthe other principal component of grains in a diffuse cloud environment.

38 Chapter 3. Molecular Hydrogen Formation on Amorphous Olivine3.1 A brief survey of previous studies and experiments3.1.1 Pre-Syracuse eraThe problem of the formation of molecular hydrogen in space has been addressed by agreat many of theoretical work. Gould & Salpeter (1963) proved the inefficiency of H 2recombination in space and were the first that considered the grain-surface route to H 2formation in space plausible; Williams (1968) investigated the mechanisms of physicaladsorption on interstellar graphite grains by atomic H and other several speciesof atoms; Hollenbach & Salpeter (1970) developed a quantum mechanical model todetermine the surface mobility and recombination efficiency of two H atoms on a grainsurfaces, and then the same authors (Hollenbach & Salpeter 1971) calculated the efficiencywith which two H atoms could recombine on a grain surface; Hollenbach et al.(1971) gave the following expression for the recombination rate of molecular hydrogenon cosmic dust grains:R H2 = 1 2 n Hv H n g σSγ. (3.1)As we have seen earlier in §1.4.2, in Eq. 3.1, n H and n g are the number densityof H atoms and dust grains, respectively, and v H is their relative velocity. S (thesticking coefficient) is the probability that an atom hitting a grain gets adsorbedon the surface, and γ is the probability that an atom, once adsorbed, makes anencounter with another H atom and recombines with it. By calculating the formationrate constant R to be ∼ 3 × 10 −17 cm 3 s −1 (equation 1.3), on the ground of UVobservations toward diffuse lines of sights, Jura (1975) estimated that, on average,Sγ ∼ 0.3. Goodman (1978) reconsidered H 2 formation on graphite grain surfaces bycalculating quantum tunnelling and thermal hopping; Smoluchowski (1981, 1983) dida quantum mechanical calculation and found that H would get trapped in the deepestsites of an amorphous surface after a few hops. Thus he obtained an efficiency of H 2production much smaller than Hollenbach & Salpeter (1971)’s. Aronowitz & Chang(1980, 1985) determined the binding energies between H atoms and different surfacesof astrophysical importance. Duley & Williams (1986) proposed a mechanism bywhich H 2 is formed on interstellar amorphous silicate grains. According to their modelnewly formed H 2 was stabilised by transferring the excess energy to a surface band.Pirronello & Averna (1988) and Averna & Pirronello (1991) investigated the possibilitythat H 2 could be produced in dense clouds by cosmic-ray particles impinging grainmantles. Of course, many other theoretical works concerning mechanisms of catalyticformation of H 2 were carried out and I cannot mention here but a few, i.e., Buch &Zhang (1991), Sandford & Allamandola (1993), Duley (1996), Takahashi et al. (1999),Farebrother et al. (2000).Nevertheless, due to experimental difficulties in reproducing interstellar conditions,

3.1. A brief survey of previous studies and experiments 39only a few experimental attempts have been carried out in order to measure the efficiencyof the catalytic process. Some pioneering work was performed in the early1960s by King & Wise (1963). Their measurements, though, suffered from experimentallimitations caused, in particular, by poor vacuum conditions. Later, Schutte et al.(1976) performed H 2 recombination measurements on the semiconductor surface of abolometer using a flux of H atoms about two orders of magnitude higher than thefluxes used in recent experiments. They obtained that at a surface temperature of∼ 3 K the hydrogen recombination efficiency (i.e., Sγ) on a surface with a previouslydeposited layer of H 2 is about 0.25, and it drops to 0.05-0.1 on a H 2 -free surface. Thesemeasurements too were affected by limitations from an astrophysical point of view.These limitations are mainly related with a) the type of surface of no astrophysicalinterest since they used a semiconductor; b) the choice of the surface temperature(∼ 3 K) that is “far” too lower than the range 10-15 K attributed to dust grains indenser clouds, and especially because in this range of interstellar temperatures theeffect of a finite residence time of H ad-atoms (i.e., adsorbed atoms) becomes important;and c) the use of a high H-atom flux that certainly hindered the investigationof a low H ad-atom coverage that characterises interstellar grains.3.1.2 Syracuse eraOnly very recently the problem of the formation of molecular hydrogen has beenaddressed experimentally on realistic dust grain surfaces and in conditions close tothose encountered in space. In fact, from 1996, systematic experimental studies onmolecular hydrogen (actually molecular HD) formation have been reported for olivine(a poly-crystalline metallic silicate), amorphous carbon and two types of amorphousice by our group headed by Pirronello and Vidali (Pirronello et al. 1997a,b, 1999,Manicò et al. 2001, Roser et al. 2002, 2003). Bare silicate and carbonaceous surfacesare very relevant for H 2 formation in the diffuse interstellar medium, while amorphousices are of interest for denser clouds where dust grains are likely to be covered byicy mantles. From these experiments, three pieces of information are obtained: therecombination efficiency (i.e., the probability of formation of molecular hydrogen whentwo atoms strike a surface), the kinetics of reaction and desorption of HD, and thevalues of the activation energy barriers. In addition, these experiments providedfor the first time a large set of data to benchmark existing theories. For example,Katz et al. (1999) and Cazaux & Tielens (2004) developed a model in relation to H 2formation on olivine and amorphous carbon, Perets et al. (2005) as to amorphous ice.Here follows a review of the past results accomplished by our group at SyracuseUniversity.

40 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineH 2 formation on olivine and amorphous carbonFor details on H 2 formation experiments on poly-crystalline olivine refer to Pirronelloet al. (1997a,b), while refer to Pirronello et al. (1999) as to the experiments on amorphouscarbon.HD Desorption rate (arbitrary units)6004002000Amorphous Carbon192 sec H+D @ 7K96 sec H+D @ 7K48 sec H+D @ 7K24 sec H+D @ 7K7 14 21 28Sample temperature (K)Figure 3.1: HD desorption rate of an amorphous carbon sample during TPD runs followingadsorption of H and D atoms with exposure times given in the legend. The adsorptiontemperature is 7 K.HD Desorption rate (arbitrary units)15010050Poly-crystalline olivine(high coverage)8.0 min H+D @ 6K5.5 min H+D @ 6K2.0 min H+D @ 6KHD Desorption rate (arbitrary units)15010050Poly-crystalline olivine(low coverage)0.55 min H+D @ 6K0.25 min H+D @ 6K0.10 min H+D @ 6K0.07 min H+D @ 6K06 8 10 12 14 16 18 20 22Sample temperature (K)06 8 10 12 14 16 18Sample temperature (K)Figure 3.2: Desorption rate of HD during thermal desorption runs from a poly-crystallineslab exposed to H and D beams at 6 K. Left graph: high coverages produce a first-orderkinetics. Right graph: low coverages manifest a second-order kinetics, i.e., peaks shift to alower temperature as coverage increases.The production yield of HD was measured for different sample temperatures (5 to20 K) both during and after irradiation with H and D beams. TPD techniques wereused to obtain HD yield after the irradiation time. The efficiency Sγ of the process ofH 2 recombination, which is proportional to the ratio of the HD yield and the numberof H and D atoms sent to the sample, was computed by taking into account correctionsfor the instrumental response and probability that H and D also produce a certain

3.1. A brief survey of previous studies and experiments 41amount of H 2 and D 2 . The recombination efficiency obtained was as high as 0.35 forolivine and 0.5 for amorphous carbon at the lower irradiation temperatures of 5 - 7 K,but dropped to significantly lower values at temperatures of interstellar importance,namely, to ∼ 0.1 for olivine and ∼ 0.2 for amorphous carbon at 10 K.As in the case of olivine, at lower sample temperatures, most of HD detected isformed because of thermal activation during heat pulse. Only a small fraction of HD isformed during the irradiation process, showing that, at least under these experimentalconditions, prompt-reaction mechanisms (Duley 1996) or fast tunnelling (Hollenbach& Salpeter 1971) are not important.However, the main result of these first experimental investigations was accomplishedby analysing the kinetics of the desorption as a function of the coverage ofH ad-atoms on the surface. As shown in Fig. 3.1, the positions of the peak maximathat shift toward lower ramp temperatures (hence to shorter times from the beginningof the temperature ramp) clearly show that H and D atoms are still present on thesurface when the warm-up begins. Hydrogen and deuterium ad-atoms are able toovercome the energetic barrier for diffusion and are set in motion at around ∼ 10K;shortly thereafter, H and D ad-atoms start diffusing and encountering one another,then recombine and leave the surface as HD, thus producing the peak signal in thequadrupole mass spectrometer. In this sense, the kinetics of the process is said to beof the second order because the production and desorption rates of the final productsare proportional to the product of the surface density of the adsorbates. The higherthe coverage of H and D atoms, the shorter the average time before they encounterand form HD molecules. This evidence is even clearer in the case of olivine (Fig. 3.2)where a lower coverage could be obtained. From this type of measurements we mayconclude that tunnelling does not favour high mobility —at least on poly-crystallineand amorphous surfaces— as it had been assumed following the influential papersof Hollenbach & Salpeter (1970, 1971), and that thermal activation is required instead.This experimental evidence allowed Pirronello et al. (1997a) to propose a newexpression for the formation rate of molecular hydrogen in interstellar clouds:R H2 = 1 2 (n Hv H σSt H ) 2 n g αγ ′ . (3.2)In this new expression of R H2 , a few new terms appear in respect to Eq. 3.1: t Hrepresents the residence time of adsorbed H atoms on the surface, α is hopping rate ofa single H ad-atom, and γ ′ here also takes into account the existence of an activationenergy for recombination. Besides, notice that the quantity squared (i.e., n H v H σSt H ) isthe average number N of H atoms adsorbed on the surface at a given time, namely, thecoverage. It is remarkable that, according to Eq. 3.2, the formation rate of molecularhydrogen is proportional to the square of the coverage of H ad-atom. Thus, it isqualitatively different from the one obtained by Hollenbach & Salpeter (1971), whichis independent of the coverage.

42 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineThe relation given in Eq. 3.2 was deduced by Pirronello on the ground of experimentalevidence that a second order kinetics occurred. Experiments provided proofthat atoms, at low adsorption temperatures, remained adsorbed without diffusing,thus, at the beginning of the thermal desorption, only the atomic reaction partnerswere present on the surface since no molecules had yet formed.The quadratic dependence of R H2 can be easily understood from the followingconsiderations. If α represents the mobility (from experimental evidence it can beassumed that mobility is due to “thermal hopping” or the so-called “thermal assistedtunnelling”) it can also be considered as the number of “hops” per second executedby an adsorbed H atom between two different adsorption sites. If N is the number ofadsorbed atoms on the surface, i.e., the coverage, then the probability that a hoppingH atom encounters a second adsorbed H atom per unit time is α(N-1). To extend thisto all the H atoms adsorbed on a grain and obtain the total number of encountersper second we have to multiply again by the total number of adsorbed atoms N, thuswe have N(N-1)α. This probability, multiplied by γ (probability that two atoms thatcome to the same adsorption site recombine) and 1/2 (two atoms are needed to makean H 2 molecule), gives the number of hydrogen molecules produced on the surface of asingle dust grain per second. Multiplying again by n g (number density of dust grainsin the cloud), we have the H 2 formation rate per unit volume and and unit time.Biham et al. (1998), through a simple model based on rate equations (discussedin the next section), demonstrated that the two expressions of the steady-state H 2formation rate are both correct. They found that Eq. 3.1, given in Hollenbach et al.(1971), applies to regimes of fast mobility and/or high coverage, while Eq. 3.2, givenby Pirronello and published in Pirronello et al. (1997a), applies to regimes of slowmobility and/or low coverage.The modelIn this section I describe the “rate equation” approach used by Biham et al. (1998)to build a simple model whose exact solution in steady state conditions gives, asparticular cases, the expression found by Hollenbach et al. (1971) and the one deducedin Pirronello et al. (1997a).The number N H of hydrogen ad-atoms per second on a grain increases becauseof the arrival of H atoms in the gas phase with flux f = n H v H and decreases owingto evaporation of H atoms with probability per unit time p (inverse of the residencetime) and to the release of H 2 molecules with a probability per unit time α. Hencewe have:dN Hdt= fSσ − pN H − αN 2 H. (3.3)The number of H 2 molecules produced by a single grain per second (assuming

3.1. A brief survey of previous studies and experiments 43prompt desorption in the gas phase upon formation) isr H2 = 1 2 αN2 H. (3.4)In steady state conditions, that we can reasonably assume in interstellar clouds,dN Hdt= 0. (3.5)Therefore Eq. 3.3 becomes an algebraic second-degree equation whose only physicallymeaningful positive solution is:and substituting it in Eq. 3.4 we getN H = −p + √ p 2 + 4αfSσ2α,r H2 = 1 2 α−p + (p2 + 4αfSσ) 1/22α== p2 + 2αfSσ − p(p 2 + 4αfSσ) 1/2. (3.6)4αThis last equation represents the “exact” solution of the problem in steady stateconditions per grain and per unit time. Thus, in the two limit cases:a) p 2 ≪ 2αfSσ, in which the evaporation rate of H atoms (as atoms) is negligiblecompared to their recombination rate on the surface we haver H2 = 1 2 fSσ −→ 1 2 n Hv H σS. (3.7)That is exactly the expression given by Hollenbach et al. (1971).( ) 1/2b) p 2 ≫ 2αfSσ, expanding the term 1 + 4αfSσp according to the series2(1 + x) 1/2 ∼ 1 + x − x2 , 2 8( ) 2 fSσα −→ 1 2 (n Hv H σSt H ) 2 α. (3.8)r H2 = 1 2pThat is exactly the expression proposed by Pirronello et al. (1997a).Expressions 3.7 and 3.8 thus show that both the formation rate given by Hollenbachet al. (1971) and the one, deduced from experiments by V. Pirronello on a purelyintuitive basis, were right but relatively to different mobility regimes and/or coverage.

44 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineH 2 formation on amorphous water iceThe experiments on water ice were performed on two different types of amorphous ice:high-density amorphous ice, obtained by depositing water molecules on a surface at 10K, and low-density amorphous ice obtained by heating high-density ice past around36 K (Jenniskens et al. 1995). Because of a wider distribution of energy barriersfor desorption of H 2 , the features of the desorption curves from amorphous ice arebroader and it is harder to separate effects due to the morphology of the surfacefrom the signature of the desorption kinetics. In Fig. 3.3 are shown traces of thermaldesorption experiments at progressively larger doses of H and D. These traces showFigure 3.3: HD yield during thermal programmed desorption runs from low densityamorphous water ice after irradiation at a surface temperature of 10 K.that HD comes off at a higher temperature than from the surface of olivine and over awider range of temperature. The low temperature peaks shift leftward as the exposureis increased, a sign that the reaction process follows a second-order kinetics. However,this effect is not as apparent as in the case of olivine, perhaps because the surface ofwater ice is much more open and disordered.In relation to the experiments performed on amorphous ice, our group also carriedout an experiment to determine the distribution of kinetic energies of hydrogenmolecules forming and desorbing from the sample surface. An amorphous water icewas actually exposed to H and D atoms; the samples was then turned towards thetime-of-flight line and a TPD was started. The molecules entering the time-of-flightline were chopped by a high speed slotted wheel. The molecules emerging from thesurface had a Maxwell-Boltzmann distribution of velocities with an average kineticenergy corresponding to a surface temperature of ∼ 24 K. The interpretation of suchresult is that HD forms in the porous structure of amorphous ice and the desorbing

3.1. A brief survey of previous studies and experiments 45HD molecule makes enough collisions as it leaves the surface that it thermalises losingmost of its (possible) initial kinetic energy (Roser et al. 2003).Fig.3.4 recaps and compares the recombination efficiency obtained for poly-crystallineolivine, amorphous carbon, and high-density and low-density ice.0,60,5Recombination Efficiency0,40,30,20,10,06 8 10 12 14 16 18 20Temperature at Irradiation (K)Figure 3.4: H 2 recombination efficiency on the major dust grain surface analogues studieduntil 2003. Filled circles (•) are for high-density amorphous ice; filled squares ( ) are forlow-density amorphous water ice (obtained by heating high-density ice); filled triangles ()are for water vapor-deposited low density ice; open squares (□ ) are for amorphous carbon,and open circles (◦) are for poly-crystalline olivine (Vidali et al. 2005).These past experimental studies demonstrated the viability of the basic processby which hydrogen molecules can form on cold surfaces. Also, these experimentalresults were used in theoretical works by Katz et al. (1999), concerning the olivine andamorphous carbon samples, and by Perets et al. (2005), concerning the amorphous icessamples, to perform fits of the experimental desorption traces. The main parametersof these fits were the activation energy barriers for atomic H diffusion and atomic andmolecular H desorption. Their results are set out in the following table.E diff,H (meV ) E des,H (meV) E des,H2 (meV)Olivine 24 32 27a-Carbon 44 57 47HDA ice 55 62 47LDA ice 44 52 69However, the situation in interstellar clouds is that the surfaces are those of verysmall, discrete dust grains, not macroscopic laboratory samples. In space, the gas

46 Chapter 3. Molecular Hydrogen Formation on Amorphous Olivinedensities are low enough that the mean population of H atoms per grain is very small.In these circumstances, a statistical approach is required to obtain the actual H 2recombination efficiency under ISM conditions; this statistical solution of molecularformation in space has been addressed, for example, by Biham et al. (2001) who, bythe way, closely collaborate with our group.3.2 Experiments on amorphous olivineFigure 3.5: SEM images of the 100% Fe amorphous olivine sample (batch #1, #2) seenat 750× (left) and at 2000× (right). This sample is one of those labelled “uniform” afterexamination at the optical microscope. By courtesy of Herma Cuppen & Eric Herbst (OhioState University).Figure 3.6: SEM images of the 50% Fe amorphous olivine sample (batch #1, #5) seenat 1000× (left) and at 2000× (right). This sample is one of those labelled “patchy” afterexamination at the optical microscope. By courtesy of Herma Cuppen & Eric Herbst (OhioState University).3.2.1 Description of the samples employedThree main classes of solids are relevant to processes occurring in interstellar space:ices, carbon-based materials, and silicates. Their chemical composition has been identifiedmainly thanks to absorption and emission spectroscopy at infrared wavelengths.

3.2. Experiments on amorphous olivine 47Figure 3.7: AFM images of sample #3 (batch#2). It shows a predominantly ruggedamorphous surface at nanometer scale. By courtesy of Herma Cuppen & Eric Herbst (OhioState University).Figure 3.8: AFM images of sample #7 (batch#2). It shows a smoother and more uniformsurface morphology than sample #3’s. By courtesy of Herma Cuppen & Eric Herbst (OhioState University).In this work, we studied experimentally the problem of molecular hydrogen formationon diverse samples of amorphous silicates. These materials represent the mostrealistic grain-surface analogues on which H 2 may form in the diffuse cloud environment.In fact, silicates are detected in the spectra of a variety of dusty sources(Mathis 1990) by the typical infrared bands 10 and 20 µm. These bands are due toSi-O stretching and O-Si-O bending modes in SiO 4 tetrahedra, the building blocks ofsilicate compounds.The classes of silicates of major relevance to the astrophysical context are olivines,(Mg x ,Fe 1−x ) 2 SiO 4 , with end-members forsterite (x=1) and fayalite (x=0), and pyroxenes,(Mg x ,Fe 1−x ) 2 SiO 3 , with end-members enstatite (x=1) and ferrosilite (x=0).Though a crystalline component has been detected in certain cosmic objects, theinterstellar silicate dust grains are predominantly amorphous (Mathis 1996, Draine2003).The dust analogues used in our experiments are amorphous olivines of the afore-

48 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineTable 3.1: Summary of characteristics of amorphous silicate samples used in the experiments.Batch Sample Fe/Mg Surface H+D SEM/AFM# # Content Condition Recombination Analysis1 1 100/0 uniform low no1 2 100/0 uniform low yes1 4 75/25 uniform low no1 5 50/50 patchy high yes1 6 50/50 uniform low nouniform1 8 25/75 (patchy high noafter removal)2 1 0/100 uniform low no2 2 25/75 uniform low no2 3 50/50 mixed not yesuniform/patchy used2 6 0/100 mixed low nouniform/patchy2 7 50/50 uniform low yesmentioned type (Mg x ,Fe 1−x ) 2 SiO 4 , with a relative fraction x of Mg, in respect toFe, being 1, 0.75, 0.50, 0.25, and 0. These samples were prepared by J.R. Brucatoat the INAF-Osservatorio Astrofisico di Capodimonte by laser ablation (wavelength266 nm) of MgO-FeO-SiO 2 mixture targets in an oxygen atmosphere (10 mbar). Theoptical and stoichiometric characterisation of samples produced with this techniqueis reported elsewhere (Brucato et al. 2002).J.R. Brucato provided us at first with a batch (bacht #1) of amorphous olivinesamples in which two samples of each single Mg/Fe mixture were present, namely,we had at disposal two samples of Mg 2 SiO 4 (0%Fe amorphous olivine), two samplesof (Mg .75 ,Fe .25 ) 2 SiO 4 (25% Fe amorphous olivine), two samples of 50% Fe amorphousolivine, two samples of 75% Fe amorphous olivine, and two samples of 100% Fe amorphousolivine, for a total of 10 samples. Each sample was composed by a thin layerof amorphous olivine deposited on a copper disk of 10mm in diameter. Table 3.1summarises some important information regarding the samples used.Subsequently, owing to some atypical results achieved during the first experiments(cf. § 3.2.3), that we feared were due to a too thin olivine layer so that the coppersubstrate could be in part exposed as well, J.R. Brucato provided a second batch(batch #2) in which the amorphous olivine layer was a little thicker than in samplesof batch #1. Still refer to Table 3.1 for details on samples of batch #2.

3.2. Experiments on amorphous olivine 493.2.2 Experimental methodsIn brief, I shall recap here the experimental methods already discussed in Chapter 2.1and employed in our laboratory. The measurements were made in an apparatus consistingof an ultra-high vacuum chamber that houses the sample holder and a rotatingdetector, the quadrupole mass spectrometer (QMS). The sample can be cooled by liquidhelium to ∼ 5 K as measured by a calibrated silicon diode and thermocouple placedin the back of the sample. A heater in the back of the sample is used to maintain aset temperature between 5 and 30 K during the irradiation phase of the experiment.Prior to each run of experiments, the sample is heated to ∼ 400 K at the beginning ofthe day, and in a time between two series of measurements, the sample can possiblybe heated to 250 K to desorb residual condensables on the sample surface. Hydrogenand deuterium gases are dissociated in two radio-frequency dissociation sources andthey are sent into the sample chamber via two triple differentially pumped beam lines.The use of two isotopes of hydrogen is a key features to have a high S/N ratio.The experiment consists of adsorbing hydrogen and deuterium atoms onto thesurface of the amorphous olivine sample while monitoring the amount of H 2 moleculesthat are formed. The irradiation time (exposure) is typically 2 minutes. However, theeffective irradiation is half the exposure time, since a mechanical chopper with 50%duty cycle is employed in each line.The measurements of the HD formation are done in two steps: first, we record theamount of HD that forms and comes off the surface while the sample is being dosedwith H and D atoms. Next, after dosing is completed, in a thermal programmeddesorption (TPD) experiment, the surface temperature is rapidly raised and the rateof HD molecules coming from the sample is measured. The purpose of this secondprocedure is to release from the surface HD molecules that remained trapped on thesample surface, or to accelerate the diffusion and reaction of H and D atoms if atomswere still present on the surface at the beginning of the TPD.Thus, with these experiments it is possible not only to measure the overall efficiencyof the process, but also to obtain information on which of the two mainmechanisms of heterogeneous catalysis, the hot-atom/Eley-Rideal or the Langmuir-Hinshelwood, is at work.A key aspect of this type of measurement is the use of beams with low fluxes andshort dosing times in order to make this measurement at the lowest possible surfacedensity (coverage) of reactants. This is necessary in order to come as close as possibleto the conditions of formation of molecular hydrogen in diffuse clouds or at least in aregime in which results can be extrapolated to interstellar diffuse environments.

50 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineHD dN/dT (arbitrary units)4500400035003000250020001500100050050% Fe Amorphous Olivine(batch #1, sample #5)all are 2 min H+D at ~10KJuly 1, 2004 #45July 8, 2004 #21July 8, 2004 #22July 16, 2004 #39August 17, 2004 #25010 15 20 25 30 35 40Sample Temperature (K)HD dN/dT (arbitrary units)250020001500100050050% Fe Amorphous Olivine(batch #2, sample #7)all are 2 min H+D at ~10KSept 29, 2005 #24Oct 13, 2005 #43Oct 11, 2005 #22010 15 20 25 30Sample Temperature (K)Figure 3.9: Desorption rate dN/dT vs T for the HD formation reaction on two samples ofamorphous olivine. Each figure also gives a comparison among TPD runs carried out withthe same exposure and adsorption temperature but in different times on the same sample.These data show a good consistency.3.2.3 Experimental resultsThermal desorption spectra measured after H and D exposure of two different 50% Feamorphous olivine samples at a temperature of ∼ 10 K are shown in Figure 3.9. Thedifferences in the thermal desorption spectra as a function of temperature between thesamples can be attributed to changes in the kinetics of the hydrogen recombinationreaction taking place on the surface. Also, the different shape of the traces, and thepossible presence of two desorption peaks for the desorption spectra, can be attributedto the variety of populations of binding sites on the two surfaces.Fig. 3.9 is also a comparison of HD desorption traces taken on the same sample#5 of batch #1 (left graph), for which a large data set is available, and sample #7 ofbatch #2 (right graph). These comparisons show that results are very reproducible.This consistency was also found for most of other samples. Shown in Fig. 3.10 arethermal desorption traces measured after H and D exposures at a sample temperatureof ∼ 10 K of two different 50% Fe amorphous olivine samples (2-minute exposure to Hand D). The similarity of TPD spectra measured between the two samples indicatessimilar hydrogen recombination kinetics and is indirect evidence that the two sampleshave similar structures.Besides, compared to previously published desorption traces from poly-crystallineolivine, a characteristic feature of the thermal desorption spectra taken from amorphousolivine samples is the much larger width of the thermal desorption peaks. InFig. 3.11 are reported TPD traces from one of the amorphous olivine samples andfrom the poly-crystalline olivine sample. The desorption trace given by the amorphoussurface is far broader than that given by the poly-crystalline olivine. Also, itis apparent that the maximum of the amorphous olivine is at a higher temperature.From this fact we can infer that a distribution of sites exists, different for each single

3.2. Experiments on amorphous olivine 51Figure 3.10: Comparison of desorption traces for the HD recombination reaction ontwo different 50% Fe amorphous olivine samples from batch #1, with H and D previouslyexposed for 2 minutes to sample at 10K. Maximum value of each trace was normalised to 1for clarity.type of surface, in which H atoms adsorb, diffuse and react leading to the recombinationof H 2 . Moreover, we can also conclude that the centre of this distribution ofsites for amorphous olivine has an activation energy barrier that is higher than thatfor poly-crystalline olivine. In section 3.3 I shall present a preliminary fit, calculatedby one of our group, of the experimental results obtained in this work.In passing, it must be noticed that very first determination of H recombinationefficiency on a polished poly-crystalline olivine surface (Pirronello et al. 1997a,b) raisedthe problem that H 2 could not form efficiently except only in an exceedingly narrowtemperature range that was lower than the estimated temperature range of grains indiffuse clouds. However, recent calculations based on data of H 2 formation on polycrystallineolivine and amorphous carbon show that the temperature range over whichefficiency is high widens if surface heterogeneity is introduced in the models (Changet al. 2005, Cuppen & Herbst 2005).Another demonstration of the range of desorption energy barriers in the case ofdiverse amorphous olivine samples is shown in Fig. 3.12, where a set of desorptiontraces was measured with the same H and D exposure but with progressively higheradsorption temperatures. The thermal desorption traces illustrate that the occupationof binding sites by adsorbates depends upon the surface temperature, either througha difference in the sticking coefficient of different binding sites, a difference in theaverage residence time for which adsorbates remain bound to the binding sites, ordiffusion of particles into the sites with the higher energy barriers before desorption.

52 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineFigure 3.11: Comparison of thermal desorption traces after irradiation with H+D on anolivine poly-crystalline sample (red circles) at 6.5 K (Pirronello et al. 1997a,b) and on a100% Fe amorphous olivine (Fe 2 SiO 4 ) at 8 K (black squares). Similar results were obtainedwith a (Fe 0.5 ,Mg 0.5 ) 2 SiO 4 sample.Recombination efficiencyThermal desorption peaks for HD are always observed when the amorphous olivinesamples are irradiated either with H and D exposures at low temperatures or withwith only D exposures at low temperatures.The method practised in the laboratory for acquiring data to determine the recombinationefficiency is to conduct two desorption measurements for each adsorptiontemperature T ads : one adsorption is with a given exposure time (usually 2 to4 minutes) of only D atoms; the other is the same exposure to D atoms but witha simultaneous exposure to H atoms (i.e., H+D irradiation). The desorption peaksobtained after H+D exposure almost invariably has a larger area than the desorptionpeak with only D exposure.Two desorptions were necessary because, for all the samples, we noticed that thereis a significant contribution to the recombination probably due to the presence of Hatoms on the surface, namely, we observed HD yield even in the cases when only D wasirradiated onto the sample. First, to be sure that no “unwanted” processes were goingon, we performed a recombination test demonstrating that there is no contributionarising from the OFHC copper shield around the sample. We also noticed that thecontribution to HD yield from D-only irradiation diminishes —relatively to the onedue to H and D irradiation— as the adsorption temperature of the sample is increased.We interpret this fact as indicating that, at least for the D atoms adsorbed in thelower energy binding sites (see discussion below), the mobility of the adsorbed D

3.2. Experiments on amorphous olivine 53HD dN/dT (arbitrary units)4000 50% Fe Amorphous Olivine(batch #1, sample #5)35004 min H+D @ 13.1K4 min H+D @ 16.0K30004 min H+D @ 18.2K4 min H+D @ 20.6K25004 min H+D @ 25.2K200015001000500HD dN/dT (arbitrary units)60005000400030002000100025% Fe Amorphous Olivine(batch #1, sample #8)2 min H+D @ 8.3K2 min H+D @ 10.4K2 min H+D @ 13.0K2 min H+D @ ~17K2 min H+D @ 19.8K2 min H+D @ 23.5KHD dN/dT (arbitrary units)014 17 20 23 26 29 32 35 38 41 44Sample temperature (K)4000100% Fe Amorphous Olivine(batch #1, sample #1)2 min H+D @ ~8.4K2 min H+D @ 10.5K30002 min H+D @ 13.5K2 min H+D @ ~16.4K2 min H+D @ 19.6K20001000HD dN/dT (arbitrary units)08 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40Sample Temperature (K)2500 50% Fe Amorphous Olivine(batch #2, sample#7)2 min H+D @ 10.5K2 min H+D @ 15.5K20002 min H+D @ 18.6K2 min H+D @ 20.6K15001000500010 12 14 16 18 20 22 24 26 28 30 32 34 36Sample temperature (K)010 15 20 25 30Sample Temperature (K)Figure 3.12: Thermal desorption traces of molecular HD after irradiation of H and Don a set of four amorphous silicate samples at different values of the sample temperatureduring irradiation. The width of the peaks is indicative of the presence of a range of energyvalues for the reaction.atoms increases as the adsorption temperature increases; thus, the most mobile Datoms will have an increased probability of reacting with each other to form D 2 andescaping detection when mass 3 is measured during the desorption phase.The contribution due to the H/H 2 present on the surface is essentially the samefor a bare OFHC copper sample and for an amorphous olivine surface. This probablyindicates that the H atoms that are picked up by the D atoms sent through the beamcome from background deposition of H or H 2 . In Fig. 3.13 are shown the contributionto HD recombination allegedly due to the H atoms on the samples when a beam ofonly D atoms is sent.One simple model to incorporate this effect introduces a new parameter f, which isthe probability that a D atom adsorbed onto the surface somehow extracts an H atomfrom the surface (instead of reacting with an atom from the beamline or desorbingwithout reacting) to form HD. The recombination efficiency is also factored as anatomic sticking coefficient S and a recombination probability γ. The new expressionfor the HD desorption yield given in § 2.2.5 would then becomeY ield HD = γS(1 − f) N HN DN H + N D+ SfN D (3.9)

54 Chapter 3. Molecular Hydrogen Formation on Amorphous Olivine0,35 Probability of HD formation in case of D-only exposures0,300,250,20100% Fe Amorphous Olivine #2 (batch #1)100% Fe Amorphous Olivine #1 (batch #1)50% Fe Amorphous Olivine #6 (batch #1)75% Fe Amorphous Olivine #4 (batch #1)Copper blank sampleS f0,150,100,050,004 6 8 10 12 14 16 18 20 22 24 26Temperature at Irradiation (K)Figure 3.13: Probability of HD formation (Sf) for every D atom in a D-only irradiationon amorphous olivine samples with continuous films.The factors Sf in Eq. 3.9 can be determined by repeating the desorption with Hand D atoms exposed to the sample with a desorption with only D atoms exposed tothe sample. In this way, Sf can be determined by calculating the HD yield from theD-only exposureSf = Y ield HDN D(3.10)Thus, for a predetermined value of γ, the values of S and f can then be determinedby a pair of desorption experiments and equations 2.11, 2.12 and 3.10. Therecombination efficiency is then calculated by removing the contribution due to theformation of HD caused by the H that is already adsorbed on the surface. In the caseof copper blank experiments, the recombination efficiency is zero, since no additionalsignal is detected when H and D are both irradiated on the surface vs the case withD-only irradiation.From all the experiments carried out on amorphous olivines, we found that therecombination efficiency during the irradiation phase is always rather small (a few percent),as shown in Fig. 3.14, compared to the desorption contribution. This indicatesthat either HD is formed and mostly remains on the surface until the temperatureis increased or HD is formed by the reaction of H and D atoms when their mobilityincreases as the temperature is raised.Calculations of the recombination efficiency for a couple of samples gave higherthan usual recombination rate, and this is also why we decided to perform otherexperiments on a new set of samples. The two samples which showed a higher re-

3.2. Experiments on amorphous olivine 550,4Recombination Efficiency (S )0,30,20,10,010 12 14 16 18 20 22 24 26Temperature (K)Figure 3.14: Recombination efficiency calculated for the 50% Fe amorphous olivine sample(batch #1, #5) for T ads > 10 K. Comparison between the contribution to the HDrecombination during the adsorption phase (open circles) and during the TPD run (filledcircles).combination were sample #5 (50% Fe amorphous olivine) and sample #8 (25% Feamorphous olivine), both from batch #1. When one of these two samples (sample#5) was examined with the SEM, it looked “patchy” and with the surface coveredmostly with silicate clusters. It was also checked whether the copper substrate wasvisible, but this was not the case and thus no interaction between the substrate andH/D atoms is thought to have occurred. SEM examinations revealed that the scale ofirregularity of the surface (clusters dimension) is of about 40nm (see Fig. 3.6). AFMscans were not possible because of the large height variation across the surface andthe fragility of the deposited silicate layer.The recombination efficiencies obtained for the other samples, together with resultsconcerning the samples of batch #2, were essentially close and consistent witheach other, although significantly lower than the recombination efficiency found forsample #5 and #8 of batch #1. In Fig. 3.15 are collected all the recombination efficienciesSγ found for each sample in the experiments performed from January 2004to June 2006. Recombination efficiency for poly-crystalline olivine too was includedfor comparison. The amorphous samples for which the recombination efficiency islow looked continuous under the optical microscope. Some of these samples were alsoexamined using the SEM and the AFM: the surface appeared much smoother than inthe case of sample #5 discussed above (refer to Fig. 3.5-3.8). Unfortunately, for the“patchy” sample examined at the AFM (#3 of batch #2) there is no data availablesince it was scratched and then could not be studied via TPD experiments.Therefore, it is reasonable to believe that the higher recombination in samples #5and #8 of batch #1 is genuine and due to the highly disordered morphology of their

56 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineRecombination Efficiency (S )1,0 100% Fe Amorphous Silicate 50% Fe Amorphous Olivine(batch #1, sample #2) (batch #1, sample #6)March 9, 2004 March 7, 2005March 29, 2004 March 11, 20050,950% FeAmorphous Olivine 25% Fe Amorphous Olivine(batch #1, sample #5) (batch #1, sample #8)June 23, 2004 March 23, 20050,8July 1, 200475% Fe Amorphous OlivineJuly 15, 2004 (batch #1, sample #4)Dec 20, 2004 (4 min exp) April 4, 2005100% Fe Amorphous Olivine 50% Fe Amorphous Olivine0,7(batch #1, sample #1) (batch #2, sample #7)Feb 24, 2005 Sep 29, 2005Feb 25, 2005 Oct 11, 200525% Fe Amorphous Olivine0,6(batch #2, sample #2)March 31, 20060% Fe Amorphous Olivine(batch #2, sample #6)0,5June14, 2006Poly-crystalline OlivinePirronello et al. 1997a,b0,40,30,20,10,06 8 10 12 14 16 18 20 22 24 26 28 30Temperature at Irradiation (K)Figure 3.15: Recombination efficiency on all amorphous olivines studied. For experimentscarried out by dosing the sample with H and D atoms, the recombination efficiency ofmolecular hydrogen is obtained by integrating the TPD traces and dividing them by halfof the dose of the impinging atoms. A correction to take into account that H and D atomscan form also H 2 and D 2 is applied. The error bars are due to statistical uncertainties inthe peak areas and beam intensities.surface. This is also confirmed by the fact that amorphous surfaces (e.g., carbon,water ice, and olivine in this work) do yield higher recombination efficiencies thansmoother ones.In relation to the consistency of results, it is remarkable that the uniform surfacesamples of batch #2 for which we studied the recombination gave similar recombinationefficiencies as most of the samples of batch #1 that looked uniform as well,independently of their Fe/Mg content. Moreover, from our large set of experiments onseveral types of amorphous olivines, we may say that the Fe/Mg ratio of the olivinesamples does not seem to affect the H 2 recombination efficiency.In conclusion, the fact that we obtained similar results for all classes of dustgrain analogues (results summarised in Fig. 3.16), indicates that the processes thatdominate adsorption and diffusion are due to weak physical adsorption interactions.

3.2. Experiments on amorphous olivine 57Recombination Efficiency0,70,60,50,40,30,20,10,0Poly-crystalline OlivineAmorhous CarbonHigh Density H 2O iceLow Density H 2O ice (heated)Low Density H 2O ice (vapor-deposired)Amorphous Olivine (high eff. group)(batch #1, sample #5)Amorphous Olivine (low eff. group)(batch #1, sample #4)6 8 10 12 14 16 18 20 22 24 26 28Temperature at Irradiation (K)Figure 3.16: H 2 recombination efficiency for all major classes of dust grain analoguesstudied at SU. Amorphous olivine samples (filled red signs) studied in this work.Desorption mechanisms and kineticsShown in Fig. 3.17 is a set of thermal desorption traces with progressively larger H andD exposures of a group of amorphous olivine samples with an adsorption temperaturevarying from 7 to 10 K. Although trailing edges suggest first-order desorption, theslight shift of peaks is indicative of second-order desorption. The second-order kineticsfeatures are certainly partially hidden by the broad curves. This is due to a widedistribution of energy barriers for desorption of HD which is, in turn, caused bythe complex morphology of the amorphous olivine surface. However, for very lowcoverages, the reaction process that leads to formation of HD follows clearly a secondorderkinetics, as apparent in Fig. 3.18. Here it is evident the symmetry of thedesorption peaks and how they shift leftward as the exposure is increased. These arehallmarks of second-order reaction kinetics, which entails that atoms diffuse and thenreact, yielding a desorption rate proportional to the probability of encountering eachother which is quadratic in the reactant’s density.In relation to the desorption spectra of Fig. 3.17, we can also notice that thedesorption yield is apparently a linear function of the exposure time of H and Datoms. Although no very long exposures were performed, irradiation times between 2and 8 minutes can be compared to the case of poly-crystalline olivine shown in Fig. 3.2(left graph). As a matter of fact, while for amorphous olivine dosed with H and Dwith exposures up to 8 minutes still a second-order desorption can be observed, thisis not the case for poly-crystalline olivine, which already begins to show a first-orderdesorption kinetics. This indicates that the amorphous olivine surface must have amuch larger surface area than poly-crystalline olivine, which has certainly a smoother

58 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineHD dN/dT (arbitrary units)10000800060004000200050% Fe Amorphous Olivine(batch #1, sample #5)8 min H+D @ ~10K4 min H+D @ ~10K2 min H+D @ ~10K1 min H+D @ ~10K30 sec H+D @ ~10KHD dN/dT (arbitrary units)12000100008000600040002000100% Fe Amorphous Olivine(batch #1, sample #1)8 min H+D @ 8.2K4 min H+D @ 8.2K2 min H+D @ 8.2K1 min H+D @ 8.3K30 sec H+D @ 8.0KHD dN/dT (arbitrary units)010 12 14 16 18 20 22 24 26 28 30 32 34 36 38Sample Temperature (K)0% Fe Amorphous Olivine35000(batch #2 sample #6)16 min H+D @ 7.1K300008 min H+D @ 7.1K4 min H+D @ 7.1K250002 min H+D @ 7.1K1 min H+D @ 7.1K2000015000100005000HD dN/dT (arbitrary units)010 12 14 16 18 20 22 24 26 28 30 32 34Sample Temperature (K)90006000300050% Fe Amorphous Olivine(batch #2 sample #7)12 min H+D @ 9.4K8 min H+D @ 9.4K4 min H+D @ 9.4K2 min H+D @ 9.4K1 min H+D @ 9.6K06 8 10 12 14 16 18 20 22 24 26 28 30 32 34Sample Temperature (K)010 12 14 16 18 20 22 24 26 28 30 32 34Sample Temperature (K)Figure 3.17: Desorption rate traces at various coverages of diverse amorphous olivinesamples. For each sample the adsorption temperature is the same, while coverage is changedin order to examine the kinetics of the HD desorption process.surface.From our experiments, we are able to extract information on the mechanism ofHD production. The use of low atom fluxes yielding a low coverage of H and D onthe surface allows us to monitor the formation of molecular hydrogen in the regime inwhich we expect that little HD be formed by the hot-atom/Eley-Rideal mechanismduring the dosing of the sample. Therefore, most of it is likely to be produced throughthe Langmuir-Hinshelwood mechanism either immediately (in the case of very highmobility) or during the TPD procedure when mobility is thermally activated. Previousattempts to recognize whether or not atomic hydrogen is still present at thebeginning of the TPD, or has been already converted into H 2 , have given contradictoryresults (Hornekær et al. 2003, Pirronello et al. 2004). In order to check that mostof the HD we detected is not formed due to the fast processes during irradiation (i.e.,because of high mobility due to tunnelling, for example) but rather due to the inducedmobility of H and D atoms during the TPD, we did the following experiment. Wefirst performed a TPD experiment after dosing a sample of amorphous olivine withmolecular HD, and compared the results with a TPD obtained after irradiating thesame sample with atomic hydrogen and deuterium; see Fig. 3.19. The different shapeof the traces and positions of the maxima suggest that either there is occupation of

3.3. Preliminary fit of the experimental results 59HD dN/dT (arbitrary units)18001500120090060030050% Fe Amorphous Olivine(batch #1, sample #5)4 min H+D @ 5.6K2 min H+D @ 5.6K1 min H+D @ 5.6K30 sec H+D @ 5.6K15 sec H+D @ 5.6K010 12 14 16 18 20 22 24Sample Temperature (K)Figure 3.18: Second-order kinetics desorption traces observed from the 50% Fe amorphousolivine sample. Notice the low coverages employed.energetically different adsorption sites in the two cases or that the rate limiting processis due to the barrier for diffusion of the reaction partners. However, I showedearlier (comments on Fig. 3.14) that during exposure of H and D to the sample, beforethe TPD, HD molecules have not, but a small percentage, yet formed. Thus, if HDmolecules formed via prompt-reaction, the TPD trace in Fig. 3.19 following exposurewith H and D (trace with peak at higher temperature) would have been showing thesame pattern and position of the peak as the TPD trace taken after dosing the samplewith HD molecules. Instead, Fig. 3.19 shows that there is a small but evident shiftin the position of the H+D desorption peak relatively to the molecular HD desorptionpeak. This suggests that a significant fraction of the HD molecules must haveformed during the TPD, namely, as the heat pulse spurred ad-atoms to diffuse andrecombine forming HD that eventually desorbs. Therefore, this demonstrates thatHD molecules recombine and leave the sample surface in appreciable amounts onlywhen the temperature is raised.In conclusion, it is also remarkable that this behaviour, together with the evidenceof a second-order kinetics, perpetuates itself in all the types of dust grain analoguesstudied so far, confirming that the expression of the formation rate of H 2 , proposedby Pirronello et al. (1997a, Eq. 3.2 in this work) and supported by the rate equationmodel by Biham et al. (1998), is the most adequate in regimes of slow mobility and/orlow coverage of ad-atoms.3.3 Preliminary fit of the experimental resultsOne of our group, Giulio Manicò at University of Catania, is developing a model tofit the experimental results of H 2 formation on amorphous olivine. Here I describe

60 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineHD dN/dT (arbitrary units)800600400200KINETICS COMPARISON50% Fe Amorphous Olivine(batch #1, sample #5)2 min HD @ 10.2K2 min H+D @ 10K010 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40Sample Temperature (K)Figure 3.19: Thermal desorption traces after irradiation with H+D atomic beams (redsquares) and molecular HD (black triangles) at a sample temperature of 10 K. The sampleis an amorphous silicate of composition Fe 2 SiO 4 . Similar results are obtained with a(Mg 0.5 ,Fe 0.5 ) 2 SiO 4 sample.his model and show what can be inferred from a preliminary fit of a desorption traceobtained in laboratory.3.3.1 The modelWe suppose a gaussian site distribution for H atoms and H 2 molecules, only a fraction1-µ of H 2 molecules desorb promptly after recombination.Be N j the number of H atoms adsorbed on a site of type j, be α j and P j the barrierfor diffusion and desorption of an H atom from a site of type j respectively. Be N ′ jthe number of H 2 molecules adsorbed on a site of type j, be α ′ j and P ′ j the barrier fordiffusion and desorption of a H 2 molecule from a site of type j respectively. Besides,let’s suppose that the number of sites of type j be N j,T . Finally let’s put N T = ∑ N j,T .For a distribution of n different sites we write the following differential equationsfor H atoms:dN jdt= F(1 − N j − N j′ )N j∑ N i− P j N j − 2α j N j − α j N j +N j,T N T N Ti≠j−α j(1 − N j,TN T− ∑ i≠j)N i + N i′ N j − ∑ N jα i N i + ∑ N j,T − N j − N j′ α i N i ,N T N T N Ti≠ji≠jwhere the term 1 takes into account the adsorption of H atoms on sites of type j,the term 2 takes into account desorption of H atoms, the term 3 takes into accountdiffusion of H atoms among sites of type j with formation of H 2 , the term 4 takes into

3.3. Preliminary fit of the experimental results 61account diffusion of H atoms from a site of type j to a different site with formationof H 2 , the term 5 takes into account migration of an H atom from a site of type j toa different vacant site, the term 6 takes into account the diffusion of an H atom froma site of type i to a site of type j with formation of H 2 , and the term 7 takes intoaccount migration to site j of H atoms of different sites. The equation includes allterms of gain and loss for the number of atoms of site j.In relation to H 2 molecules we have:dN ′ jdt= −P jN ′ j ′ N j+ µα j N j + µ ∑ N jα j N i +N T N Ti≠j−α ′ j(1 − N j,TN T− ∑ i≠j)N i + N i′ N j ′ + ∑ Nα ′ j,T − N j − N j′iN i ′ .N T N Ti≠jNow let’s convert the equations in units of monolayer. We need to divide bothterms of the equations by N T . Let’s put β = N j,TN T, Ñ j and Ñ j ′ equal to the concentrationof H atoms, and H 2 molecules in fraction of monolayer on sites of type j. Wehave:dÑjdt= ˜F(1 −Ñ j − Ñ )j′ − P j Ñ j − ∑ (α i + α j )β ÑiÑj+ii≠j−α j[1 − β j − ∑ i≠j(Ñi + Ñ ′ i) ] Ñ j + ∑ i≠jα i(β j − Ñj − Ñ ′ j)Ñi ,anddÑ ′ jdt= −P ′ jÑ ′ j + µ ∑ i≠jα i Ñ i Ñ j +−α ′ j[1 − β j − ∑ i≠j(Ñi + Ñ ′ i) ] Ñ ′ j + ∑ i≠jα ′ i(β j − Ñj − Ñ ′ j)Ñ′i .Then the rate of formation of H 2 takes the form:˜R H2 = 1 2 (1 − µ) ∑ i≠j(α i + α j )ÑiÑj + ∑ jP ′ jÑ ′ j

62 Chapter 3. Molecular Hydrogen Formation on Amorphous OlivineFigure 3.20: Preliminary fit of a HD desorption trace after 2 min H+D at 6.2 K on a75% Fe amorphous silicate sample. By courtesy of Giulio Manicò.3.3.2 A preliminary fitThe desorption trace used for the first fit is a 2-minute H+D exposure at 6.2 K onsample #4, batch #1 (75% Fe amorphous olivine). Before fitting the background wassubtracted. By fitting with the model above we were not able to reproduce completelythe shape of the peaks. So we decided to add more than one Gaussian distribution ofsites. Using five distributions we obtained the fit shown in Fig. 3.20.We obtained the raw fit starting with a single distribution and getting the best fitby varying the following parameters:E1 : desorption activation energy barrier for H (meV)rE1 : ratio between diffusion and activation energy barrier for HE2 : desorption activation energy barrier for H 2 (meV)rE2 : ratio between diffusion and activation energy barrier for H 2w: width of the Gaussian distribution (meV)µ: fraction of H 2 molecules which do not desorb after formationFrom the one-distribution fit we get:E1 38.7rE1 0.8E2 35.5rE2 0.7w 3.5µ 0.27Then we improved the fit by adding four Gaussian distributions. Height (respect

3.4. Summary and implications of astrophysicalrelevance 63to the main distribution) and position obtained are the following:Height Position0.33 47.45 meV7.4×10 −4 25.57 meV0.13 56.20 meV0.10 34.33 meVThe parameters rE1, rE2, w and µ are the same for all the distributions.In Fig. 3.21 is shown a comparison among the fitted experimental TPD traces (leftgraph) and the relative H 2 recombination efficiencies (right graph) for three classes ofcosmic dust analogues.Figure 3.21: Left graph: Comparison of fitted desorption traces of poly-crystalline olivine,amorphous silicate, and amorphous carbon. Right graph: Recombination efficiency of molecularhydrogen vs. temperature for ISM conditions obtained using parameters inferred fromexperiments. By courtesy of Giulio Manicò.3.4 Summary and implications of astrophysicalrelevanceThe experiments presented in this work give fundamental insights into the mechanismsand efficiency of molecular hydrogen recombination on one of the more realisticanalogues of dust grain surfaces in the diffuse environment. In fact, amorphous silicates,such as amorphous olivine, are one of the principal components of interstellardust grains which, in diffuse clouds, are thought to be bare and at a temperature inthe range 15 - 20 K.According to calculations made by Jura (1975), hydrogen recombination efficiencyon dust grains is assumed to be Sγ ∼ 0.3 for a typical diffuse cloud environment. Therecombination efficiency found experimentally is Sγ ∼ 0.2 between 10 and 20 K formost of the amorphous olivine sample, and ∼ 0.4 - 0.5 for a couple of samples that

64 Chapter 3. Molecular Hydrogen Formation on Amorphous Olivineshowed a higher value of Sγ (see Fig. 3.15). This, first of all, established the actualfeasibility of H 2 formation on cosmic dust analogue surfaces in simulated astrophysicalenvironments within a little wider range of temperatures (15 - 30 K) than previouslyfound on amorphous carbon (Pirronello et al. 1999). Thus, this temperature rangeover which there is significant H 2 formation in these experiments becomes directlycomparable to the range of dust grain temperatures characterising diffuse clouds,where, due to the strong UV radiation fields, the smaller dust grains most of allcan reach higher temperatures than the assumed average of 15 K. These results alsoovercome the problematic interpretation of experiments on poly-crystalline olivine,for which molecular hydrogen formed within a very narrow range of temperatures,and, in any case, below ∼ 15 K, as shown in Fig. 3.11.Also, the samples to which was attributed a higher recombination efficiency lookedto have a more complex surface structure, namely, covered with small clusters downto a scale of 40 nm. Moreover, not only we found a higher recombination efficiencyon the samples with the roughest surfaces, but we also observed that there is a littlewider range of temperature. This may have interesting implications in relation to theformation of H 2 at low optical depths in diffuse clouds (i.e., A V between 0 and 1),where only the most amorphous grains could favor the process. Therefore, we concludethat the morphology of the sample surface, and, in turn, of cosmic dust grains, playsan important role in the formation of molecular hydrogen.However, to obtain information upon the recombination efficiency in actual ISM,rate equation and master equation models must be implemented. In fact, formation ofH 2 in space takes place on grains showing a broad distribution of sizes covering muchof the range between a micron and a nanometer, and under extremely low fluxes ofH atoms. The experiments, though, play a crucial role. The high reliability of ourresults stems from the experimental conditions which we are able to reproduce. Althoughfar from interstellar environments, our studies still are conducted in a regimeof low temperature and at an estimated coverage of a small fraction (a few percent)of a monolayer on the surface of the sample. This makes our results of great interest,though not conclusive, since the parameters obtained from experimental studies oncosmic dust analogues can be inserted into the models to provide more realistic resultsupon the production rate of molecular hydrogen in interstellar clouds. Refer to Katzet al. (1999), Biham et al. (2001), Cazaux & Tielens (2004), Cuppen & Herbst (2005),Cuppen et al. (2006) about the modelling of H 2 recombination on poly-crystalline andamorphous olivine in diverse interstellar conditions.In relation to the mechanisms of H + D −→ HD reaction on amorphous olivine,experimental evidence, such as the observed second-order kinetics in the process ofHD recombination (see Fig. 3.17 and Fig. 3.18) and the difference between TPD experimentsdone with doses of H+D and HD molecules (see Fig. 3.19), suggest that

3.4. Summary and implications of astrophysicalrelevance 65the mobility of H and D ad-atoms necessary to an effective diffusion on the surfaceto form HD is given by either thermal hopping or by what can be called “thermallyassisted tunneling”, namely, the heat pulse provided during TPDs would favor tunnellingamong physorption sites. The classical tunnelling invoked by Hollenbach &Salpeter (1971) seems to be ruled out at very low temperatures, instead. Thus, ourexperiments convincingly showed that the Langmuir-Hinshelwood mechanism is atwork in the process of molecular hydrogen recombination, at least in the coverageregime reproduced in our studies. Yet, the coverage regime that we achieve in laboratoryis certainly much higher than that characteristic of interstellar dust grainsand in diffuse clouds in particular. This, we think, provides conclusive proof thatformation of molecular hydrogen in diffuse and moderately dense clouds can be wellexplained by the Langmuir-Hinshelwood mechanism, since the temperature of grainslies within the range in which mobility of ad-atoms is high. In very cold and denseregions (with temperatures below 10 K) it is the hot-atom/Eley-Rideal mechanismthat allows efficient formation of H 2 , since the H coverage of grains can reach highvalues due to the lack of desorption and thus a long residence time.In the case of astrophysical environments where the grain temperatures are higher,such as the PDRs (T grain = 15 − 100 K), the possibility of H 2 formation must thenrely on the presence of strong binding sites, namely, chemisorption sites.

66 Chapter 3. Molecular Hydrogen Formation on Amorphous Olivine

Chapter 4Construction of the FT-RAIRSfacilityAs discussed earlier, the techniques used in the laboratory of Surface Science at SU aremass spectrometry and thermal programmed desorption in a UHV chamber at cryogenictemperatures, in conjunction with the unique two-atomic/molecular-beamlineconfiguration. These tools allowed our group to accomplish important successes, noteven ever grazed by anyone else before, in one of the main questions concerning astrochemistry:the experimental investigation of molecular hydrogen formation in space.Nonetheless, laboratory surface astrophysics involves many more interesting questionregarding grain-surface reactions that surely lead to the formation of more complexmolecular species in the interstellar medium. Therefore, our group decided tointegrate the experimental apparatus with a new piece of equipment to perform surfaceinfrared spectroscopy. In particular, because surface chemistry would be themain application, the group decided to begin the construction of a facility to employthe technique called Reflection-Absorption Infrared Spectroscopy (RAIRS). RAIRSis a valuable technique to conduct studies on molecules adsorbed on a surface, andprovides informations that cannot be easily inferred from TPD data. In fact, thecombination of TPD and RAIRS provides a better understanding of the system thaneither technique alone. Besides, RAIRS will allow us investigate surface-formationprocesses concerning complex molecules, starting from rather simple water molecules,to refractory carbon bearing molecules, of astrophysical and astrobiological importance,that cannot be investigated thoroughly through the TPD technique.Within the research group, I was the one made responsible for the study of the applicabilityof RAIRS to the existent apparatus, for the design of the new configuration,and finally for the construction of the RAIRS setup.In this Chapter I introduce the characteristics and physical principles on whichRAIRS is based, and discuss, “piece by piece”, the design and the construction of theFT- (Fourier Transform) RAIRS facility.

68 Chapter 4. Construction of the FT-RAIRS facility4.1 IR SpectroscopyThe infrared (IR) region of the electromagnetic spectrum encompasses the frequenciesranging from about 3.8 × 10 14 to 3.0 × 10 11 Hertz (or about 0.78 to 300 µm inwavelengths). These frequencies correspond to the range of energies characteristic ofvibrational oscillations of molecular bonds. This entails that molecules either emitIR radiation if they are vibrationally excited or absorb IR radiation when IR lightimpinge them.The IR region, from the standpoint of both application and instrumentation, isconveniently subdivided into three classifications: the near, mid, and far IR (seeFig. 4.1), with the frequency range of 1.2 × 10 14 to 2.0 × 10 13 Hertz (2.5 to 15 µm),being the most commonly studied. This is the so called “fingerprint” region wheremost single and double bonds absorb, and where the most useful chemical informationexists.Figure 4.1: The IR region of the electromagnetic spectrum.Infrared spectroscopists very rarely use frequencies. Indeed, spectra are sometimesreported in µm, although another unit, ¯ν (wavenumber), is currently preferred.A wavenumber is the inverse of the wavelength in cm:¯ν = 1 λ(4.1)where ¯ν is in units of cm −1 , λ is in units of cm. A wavenumber represents the numberof full waves of a particular wavelength per cm of length (typically in vacuum, index ofrefraction n=1). Even though this unit may look inconvenient to many physicists, itis very common in IR spectroscopy and has the great advantage of being proportional

4.1. IR Spectroscopy 69to energy:E = hc¯ν (4.2)In wavenumbers, the mid-IR range lies between 4000 - 400 cm −1 .In IR spectroscopy, molecules are exposed to infrared radiation. In order forabsorption of infrared radiation to occur, the vibrational movement of the molecularbonds must have associated with it a fluctuating electric dipole moment; for diatomicmolecules, this requires that the charge distribution around the molecule cannot besymmetrical. In fact, homopolar species (H 2 , N 2 , O 2 , Cl 2, . . . ) are not IR active,while diatomic polar species (HCl, CO, CS, . . . ) and polyatomic species ( H 2 O, CO 2 ,NH 3 , . . . ) not only give rise to transitions at the fundamental vibration frequencies,but also to gradually weaker transitions corresponding to the second, third, and soon, overtones.The strength of the dipole moment is determined by the magnitude of the chargedifference and the distance between the centers of charge and, as the molecule bendsand stretches, the changing dipole moment establishes an electric field. When thealternating electric field of the electromagnetic radiation matches in frequency one ofthe natural oscillatory frequencies of the molecule, interaction between the two fieldsmay occur, resulting in resonant energy transfer and a change in amplitude of theoscillatory motion.There are two types of molecular vibrations, stretching and bending. A moleculeconsisting of N atoms has a total of 3N degrees of freedom, corresponding to thecoordinates (X, Y, and Z) of each atom in the molecule. In a nonlinear molecule,3 of these degrees are rotational and 3 are translational, and only the remainingcorrespond to fundamental vibrations; in a linear molecule, 2 degrees are rotationaland 3 are translational. The net number of fundamental vibrations for nonlinear andlinear molecules is therefore:molecule vibrational modesnonlinear 3N - 6linear 3N - 5The fundamental vibrations for water, H 2 O, are given in Fig. 4.2. Water, whichis nonlinear, has three fundamental vibrations. The two symmetric modes ν 1 and ν 2give rise to bands at 3652 and 1595 cm −1 respectively, while the asymmetric mode ν 3produces a band around 3756 cm −1 .Carbon dioxide (CO 2 ), still composed of three atoms, unlike H 2 O, is linear andhence has four fundamental vibrations (Fig. 4.3). The symmetric mode ν 1 of CO 2is inactive in the infrared because this vibration produces no change in the dipolemoment of the molecule. The two scissoring or bending vibrations (ν 2 and ν ′ 2) areenergetically equivalent and, therefore, have the same frequency and are said to be

70 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.2: Stretching and bending vibrational modes for H 2 O.Figure 4.3: Stretching and bending vibrational modes for CO 2 .degenerate, appearing in an IR spectrum at ∼ 666 cm −1 . The asymmetric stretch ν 3of CO 2 gives a strong band in the IR at 2350 cm −1 .Since vibrational and rotational movements in molecules are quantized, discreteenergy absorption results. The number and position of absorption peaks in the spectraare a function of many influencing factors and constitute both the interest anddifficulties in spectral interpretation. Molecular symmetries, degenerate energy levels,low absorption coefficients and absorption peaks out of range of the instrument allcontribute to “missing peaks”. On the other hand, extra peaks occur due to overtones,as well as from combination or difference frequencies. Likewise, shifts in frequencymay occur due to vibrational coupling (interactions) both within the molecule andwith adjacent molecules, and for surface-adsorbed molecules these factors may be evenfurther complicated by interactions with the surface.All these complications may be, at least in part, overcome by instruments andtechniques which can provide high spectral resolution, high spectral accuracy, andhigh S/N ratio also in presence of weak signals. In what follows, I shall describe thetheoretical principles and practical issues of FT-RAIRS, which is recognised as one ofthe most effective tools in surface IR spectroscopy.

4.2. FT-IR spectroscopy 714.2 FT-IR spectroscopyAn FT-IR (Fourier Transform Infra-Red) spectrometer is an instrument which acquiresbroadband near-IR to far-IR spectra. Unlike a dispersive instrument (i.e., agrating monochromator or a spectrograph) an FT-IR spectrometer is able to collectall wavelengths simultaneously. This feature is one of the several advantages of FT-IRspectrometers over traditional ones, called the Multiplex or Felgett Advantage.Although FT-IR spectrometers are often simply called FT-IRs, FT-IR is actually amethod of obtaining infrared spectra by first collecting an interferogram (intensity vstime) of a sample signal using an interferometer, and then performing a Fourier transformon the interferogram to obtain the spectrum (intensity vs frequency). An FT-IRSpectrometer collects and digitises the interferogram, performs the FT function, anddisplays the spectrum.4.2.1 A brief review of the Fourier Transform methodsThe Fourier transform converts waveform data in the time domain into the frequencydomain. The Fourier transform accomplishes this by breaking down the originaltime-based waveform into a series of sinusoidal terms, each with a unique magnitude,frequency, and phase. This process, in effect, converts a waveform in the time domainthat is difficult to describe mathematically into a more manageable series of sinusoidalfunctions that, when added together, exactly reproduce the original waveform.Plotting the amplitude of each sinusoidal term versus its frequency creates a powerspectrum, which is the response of the original waveform in the frequency domain.Fig. 4.4 illustrates this time to frequency domain conversion concept.Before computers, numerical calculation of a Fourier transform was a tremendouslylabor intensive task because such a large amount of arithmetic had to be performedwith paper and pencil. These calculations became more practical as computers andprograms were developed to implement new methods of Fourier analysis. One suchmethod was developed in 1965 by James W. Cooley and John W. Tukey (Bracewell1989). Their work led to the development of a program known as the Fast Fouriertransform (FFT). The fast Fourier transform is a computationally efficient method ofgenerating a Fourier transform.The FFT is a computationally rapid way to generate a power spectrum based ona 2 th -power data point section of waveform. But this also means that the numberof points plotted in the power spectrum is not necessarily as many as was originallyintended. Then, to minimise power spectrum distortion (e.g., spread out of peaks)and ensure accuracy in the frequency domain, the solution employed is to multiply thetime series by a window weighting function before the FFT is performed. However,this window may attenuate important information appearing on the edges of the timeseries to be evaluated.

72 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.4: The Fourier transform illustrated.An alternative to the FFT is the Discrete Fourier transform (DFT). The DFTallows you to adjust the end-points that define the range of the waveform to be transformed,thus eliminating the need for windowing. This approach allows a waveformcontaining any number of points to be evaluated, and this provides more flexibilitythan the fixed-length, 2 th -power FFT. However, to prevent the same distortion effectexperienced with a non-windowed FFT, the DFT must be generated over a wholenumber of periods starting at the waveforms mean level crossing. On the negativeside, versatility and precision of the DFT come at the expense of added computationtime by the algorithm.Although I shall not go into any deeper detail of the above computation techniques,it is important to point out that DFT is the one used in the research-dedicatedinstruments of interest here, namely, FT-IR spectrometers, from which high qualityspectra are required.4.2.2 Principles of FT-IR: The Michelson InterferometerInvented more than one hundred years ago, the two-beam Michelson interferometer(Michelson 1891) is still the heart of most modern Fourier transform infrared

4.2. FT-IR spectroscopy 73spectrometers. It consists of a fixed mirror, a moving mirror and a beamsplitter,as illustrated in Fig. 4.5. The beamsplitter is a laminate material that reflects andFigure 4.5: Optical diagram of a classic Michelson interferometer, which consists of threemajor components: a fixed mirror, a moving mirror and a semi-transparent mirror, theso-called beamsplitter.transmits light equally. The collimated infrared beam from the source (S) is partiallytransmitted to the moving mirror and partially reflected to the fixed mirror by thebeamsplitter. The two IR beams are then reflected back to the beamsplitter by themirrors. The detector (D) then sees the transmitted beam from the fixed mirror andthe reflected beam from the moving mirror simultaneously. The two combined beamsinterfere constructively or destructively depending on the wavelength of the light andthe optical path difference introduced by the moving mirror. The latter is referredto as retardation, δ (cm). To obtain an interferogram, I(δ), the detector signal isdigitised and recorded as a function of retardation. The interferogram intensity of apolychromatic source is mathematically described as (Griffiths & deHaseth 1986):I(δ) =∫ +∞−∞B(σ) cos(2πσδ)dσ (4.3)where B(σ) is the spectral intensity at wavenumber σ (cm −1 ).The interferogram I(δ) is a simple sinusoidal wave when a monochromatic source isused. For a continuum (or polychromatic) source, I(δ) is a superposition of sinusoidalwaves for IR light at all wavenumbers σ, as shown in Fig. 4.6. At zero path differenceor zero optical retardation (i.e., when the moving and fixed mirrors are the samedistance from the beam splitter), all the sinusoidal waves are totally constructive,producing a centerburst on the interferogram.

74 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.6: Center-burst interferogram for a polycromatic light source.Fourier transformation of I(δ) gives the single beam IR spectrum expressed asbelow:B(σ) =∫ +∞−∞I(δ) cos(2πσδ)dδ (4.4)In practice, a discrete Fourier transform (DFT) is used as a digital approximationof the continuous Fourier series. Thus Eq. 4.4 reduces to:∑N−1B(k · ∆σ) =n=0I(n · ∆δ) cos( 2πknN ) (4.5)where the continuous variables of wavenumber σ, and optical retardation δ, havebeen replaced by discrete values k and n, respectively. The summation is then overthe total number of discrete data points N.The Optical Path Difference (OPD) is the optical path difference between thebeams travelling through the two arms of the interferometer. OPD is equal to theproduct of the physical distance travelled by the moving mirror (multiplied by 2, 4,or other multiplier which is a function of the number of reflecting elements used) andn, the index of refraction of the medium filling the interferometer arms (dried air,nitrogen for purged systems, etc.).FT-IR spectrometers have a natural reference point when the moving and fixedmirrors are the same distance from the beam splitter. This condition, already mentionedearlier, is called zero path difference or ZPD for short.The moving mirror displacement, δ, is thus measured from the ZPD and we have

4.2. FT-IR spectroscopy 75that, in the configuration given in Fig. 4.5, the beam reflected from the moving mirrortravels 2δ further than the beam reflected from the fixed mirror. The relationshipbetween OPD and mirror displacement, δ, is:OPD = 2δnAs the optical path difference grows, different wavelengths produce peak readingsat different positions and, for a broadband signal, they never again reach their peaks atthe same time. Thus, as you move away from centerburst, the interferogram becomesa complex looking oscillatory signal with decreasing amplitude. Each individual spectralcomponent contributes to this signal a single sinusoid with a frequency inverselyproportional to its wavelength.4.2.3 Advantages of FT-IR instruments over dispersive instrumentsThere are three significant advantages that FT-IR spectrometers hold over dispersivespectrometers. Fig. 4.7 compares the schematic optical configurations of a dispersiveand a FT-IR spectrometer.Figure 4.7:instrument.Diagrams of a) a dispersive spectral instrument and b) an FT-IR spectralMultiplex (Fellgett) Advantage. In a dispersive spectrometer, wavenumbers areobserved sequentially, as the grating is scanned. In an FT-IR spectrometer, all thewavenumbers of light are observed at once. When spectra are collected under identical

76 Chapter 4. Construction of the FT-RAIRS facilityconditions (spectra collected in the same measurement time, at the same resolution,and with the same source, detector, optical throughput, and optical efficiency) ondispersive and FT-IR spectrometers, the S/N ratio of the FT-IR spectrum will bemuch greater than that of the dispersive IR spectrum. As an example, consider thata 2 cm −1 resolution 800 - 8000 cm −1 spectrum measured in 30 minutes on a dispersivespectrometer would be collected at equal S/N on an FT-IR spectrometer in 1 second,provided all other parameters are equal.The multiplex advantage is also shared by array detectors (e.g., CCDs) attachedto spectrographs. However, the optimum spectral ranges for these kinds of systemstend to be much shorter than that for FT-IR spectrometers and therefore the twotechniques are mostly complementary to each other.The Throughput Advantage. FT-IR instruments do not require slits to achieveresolution. Therefore, it is possible to get much higher throughput with an FT-IRspectrometer than with a dispersive instrument. This is called the Jacquinot Advantage.Actually, there are some slit-like limits in the system, due to the fact that oneneeds to achieve a minimum level of collimation of the beams in the two arms of theinterferometer for any particular level of resolution. This translates into a maximumuseable detector diameter and, through the laws of imaging optics, it defines a usefulinput aperture.High Resolution. Spectral resolution is a measure of how well a spectrometer candistinguish closely spaced spectral features. FT-IR spectrometers are capable of highresolution because the resolution limit is simply an inverse of the achievable opticalpath difference, OPD. A 2-cm OPD capable instrument can reach 0.5 cm −1 resolution.Thus, in FT-IR instruments, the maximum achievable value of OPD determines thespectral resolution. Spectral features at 2000 cm −1 and 2002 cm −1 , for example, canthen be distinguished from each other at values of OPD = 0.5 cm or longer.4.2.4 Wavelength accuracy through the HeNe laserA collimated, monochromatic light source will produce an interferogram in the formof a sinusoid. When the light intensity goes from one maximum of the interferogramto the next maximum, the optical path difference between the two arms of the interferometerchanges by exactly one wavelength of the incoming radiation. To determinethe wavelength of the incoming radiation, one way is measure the frequency ν orperiod τ = 1/ν of the interferogram with, for example, an oscilloscope. Then thewavelength can be determined through the formula:λ = V 0 · τ = V 0 /ν (4.6)

4.3. The RAIRS technique 77Where V 0 the speed of change of the optical path difference. There is, however, animportant practical difficulty: the velocity V 0 (which is in practise proportional to thevelocity of the moving morror) has to be constant at all times, and we need to knowwhat this velocity is, with a high degree of accuracy. An error in the velocity value willshift the wavelength scale according to Eq. 4.6. Fluctuations in V 0 will then manifestthemselves as deviations of the interferogram from a pure sine wave that, in turn,will be considered as a mix of sinusoids. In other words, we would think that there ismore than one wavelength in the incoming radiation. This behavior produces what arecalled “spectral artefacts”. Since the manufacture of an interferometrically accuratedrive is extremely expensive, FT-IR designers added an internal reference source intothe interferometer to solve the drive performance problem. A HeNe laser emits lightwith a wavelength which is known with a very high degree of accuracy and which doesnot significantly change under any circumstance. The laser beam parallels the signalpath through the interferometer and produces its own interferogram at a separatedetector. This signal is used as an extremely accurate measure of the interferometerdisplacement (optical path difference). In this way, the following equation for a HeNelaser based FT-IR can be used to calculate the exact wavelength of the source:λ = λ HeNe(ν HeNe/ν) (4.7)Where the subscript denotes the HeNe reference. It is straightforward now tocalculate the wavelengths with accuracy and a have a reliable spectrum without extremelytight tolerances on the velocity.4.3 The RAIRS techniqueInfrared spectroscopy was among the first techniques to be applied to the directcharacterisation of adsorbates since it is one of the few surface-sensitive probes thatprovide molecule-specific information without perturbing the chemisorbed state ofmolecules adsorbed on surfaces.What is of great interest here, are studies concerning infrared spectra obtainedunder Ultra-High Vacuum (UHV) conditions of small molecules adsorbed on surfacesof astrophysical interest. Small molecules that can be deposited from the gas phaseare also the ones that are most relevant to heterogeneous catalysis, and of course allthe experiments considered in this work are motivated by the a desire to understandsurface chemistry of catalytic importance occurring on cosmic dust grains.Here, I shall consider a particular surface IR spectroscopy technique named ReflectionAbsorption IR Spectroscopy (RAIRS). Whenever a FT-IR spectrometer is usedin conjunction with RAIRS applications, as it is almost always the case today, thistechnique is then referred to as FT-RAIRS.

78 Chapter 4. Construction of the FT-RAIRS facilityThe development of RAIRS stemmed from the definite advantage of being able toinvestigate adsorbed molecules of a gaseous species on a bulk metal without the complicationintroduced by the need to have an IR transparent and very thin substrate,as it used to be in early studies which employed transmission infrared spectroscopy.Then, since standard transmission techniques were often obviously inapplicable, attentionwas turned to reflectance methods. This consists of reflecting a beam of infraredradiation off a metal surface (through an adsorbed layer) and looking at the loss in intensityof the reflected light at frequencies which correspond to the vibrational modesof either the adsorbed species itself or as a result of the interaction of the adsorbedspecies with surface atoms of the substrate. RAIRS was first applied many yearsago, when Pickering & Eckstrom (1959) recorded spectra from carbon monoxide andhydrogen adsorbed on metal films, using a multiple reflection arrangement in whichthe infrared beam was at near normal incident to the substrate.As we shall see in the next section, however, following the principles laid downby Greenler (1966), higher sensitivity can be obtained in a reflection experiment ifgrazing angles of incidence are employed.4.3.1 Physical principlesThe interaction of infrared radiation with an adsorbed layer depends significantly onthe optical properties of the substrate, metal surface behaving in a very differentmanner from semiconductors. I shall only consider the situation for metal surfaces.Figure 4.8: Electric vectors of the s- and p-polarized components radiation incident ona metallic surface. The directions of transition dipole of the adsorbed molecules is alsolabelled. The shortening of the bar representing the reflected p-polarized light indicatesabsorption by the surface molecules oriented perpendicular to the surface.When a beam of light is reflected from a metallic substrate, the phase of thereflected beam shifts by an amount determined by the angle of incidence, the state ofpolarization, and the wavenumber of the light. As illustrated in Fig. 4.8, the phaseshift for the s-polarized beam (with electric field vector, E, perpendicular to the plane

4.3. The RAIRS technique 79of incidence, or parallel to the surface of the sample) is close to 180 ◦ , irrespective ofthe angle of incidence and wavenumber (Umemura 2001). The reflection coefficients ofmetals are close to unity in the infrared, thus the electric field vectors of the incidentand reflected beams for s-polarized light nearly cancel out at the surface. Since theIR absorption is proportional to (µ • E) 2 , where µ and E refer to the transition dipolemoment of the sample and the electric field intensity of IR light, this cancellation ofthe E vector at surface (E ≈ 0) means that little absorption is to be expected from athin film on a metallic substrate for s-polarized IR light, regardless of the orientationof the transition dipole. The phase shift for a p-polarized beam (with E parallel to theplane of incidence, or perpendicular to the surface of the sample) increases rapidly,from as small as a few degrees to the extreme 180 ◦ , as the angle of beam incidenceincreases from 80 - 90 ◦ . When the phase shift is in the range of 0 - 90 ◦ , as it is themost common case for grazing angle incidence over large mid-IR wavenumber region,the incident and reflected beams usually produce a finite field vector E perpendicularto the metal surface. Accordingly, the p-polarized beam will be absorbed by moleculeswith transition moments oriented perpendicularly to the surface.Therefore, the resultant field has a component parallel to the surface which issmall, but the perpendicular component is large at angles of incidence close to grazing.Fig. 4.9 illustrates the resultant fields for s- and p- components as a function of theangle of incidence.As stated above, the absorption intensity of a molecular dipole is proportional tothe square of the oscillating electric field component along the dipolar axis, so theforegoing discussion enables us to establish at once the principal selection rules forRAIRS at metal surfaces:• Only p-polarized radiation is absorbed to a significant extent by the ad-layer.• Only vibrations which have a component of the induced dipole perpendicular tothe surface are infrared active.The enhancement of the electric field at the surface which occurs at high angles ofincidence indicates that RAIRS should be most sensitive when angles close to grazingare are employed. An additional, geometrical factor also improves sensitivity at suchangles: at grazing incidence, the beam will cover more of the surface and so interactwith more molecules. Calculations show that the combination of these two effectsleads to a maximum sensitivity when the angle of incidence is between 85 and 88 ◦ ,depending on the optical properties of the metal and the wavenumber of interest.In practice however, an angle of ∼ 80 ◦ is often used as a compromise between thetheoretical optimum and the fact that at this angle the irradiated surface area tendsto become larger than the actual sample area. Moreover, because of the need for alarge angle of incidence the sample surface must have high reflectivity and be polished

80 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.9: Amplitudes of surface electric field components as a function of the angle ofincidence in terms of the incidence field E 0 . Fields from p- and s-polarized components areindicated by solid and broken lines respectively, while optical constants are those appropriatefor copper at 2000 cm −1 . From Hollins (1994).to a near mirror finish. As a result, only metals very reflective in the mid-IR havebeen used as substrate surfaces. Among the most common choices are gold (Au),silver (Ag{111}), copper (Cu{100}, Cu{110}, Cu{111}), platinum (Pt{111}), nichel(Ni{110}), and ruthenium (Ru{001}).4.3.2 Further remarks about RAIRSRAIRS is recognised to be a non-destructive surface technique able to probe vibrationaltransitions of species adsorbed on surfaces. Its major advantage over transmissioninfrared spectroscopy is that it can be used to detect adsorbate-adsorbateand adsorbate-surface vibrations and interactions at monolayer and sub-monolayercoverages on metals.As a consequence of the relatively low IR intensities used, the interaction with theadsorbate layer is linear in the electric field and, for the most part, a RAIRS spectrumcan be compared with the conventional absorption spectrum of the same molecule.Alternatively, spectra recorded using RAIRS can be transformed, even though not

4.4. Importing FT-RAIRS into our laboratory 81always with precise results, into transmission spectra using appropriate adaptationsof the Fresnel equations (McIntyre & Aspnes 1971) for a pseudo-direct comparisonwith IR observations.Another important advantage is that the RAIRS technique provides the abilityto obtain a reference spectrum of the adsorbate-free substrate followed immediatelyby the adsorbate-covered surface, without moving the sample, and this is key whenworking in UHV. Also, spectra can be taken at regular intervals during the experimentto track changes in the adsorbates structure and composition as they evolve and react.One major problem of RAIRS is sensitivity, i.e. the signal is usually very weakowing to the small number of adsorbing molecules. Typically, the sampled area is∼ 1 cm 2 with less than 10 15 adsorbed molecules (about 1 nanomole). With modernFT-IR spectrometers, however, such small signals (0.01% - 2% absorption) can stillbe recorded at relatively high resolution (around 1 cm −1 ).4.4 Importing FT-RAIRS into our laboratory4.4.1 Preliminary study of the projectThe first step toward the importation of a FT-RAIRS facility into the existent apparatus,described in detail in Chapter 2.1, was an evaluation of the actual feasibility ofsuch a project.In order to apply the RAIRS technique to samples kept inside the UHV chamber, itwas necessary to send (and than to collect) the IR beam to (from) the sample from theexterior side of the chamber, and arrange the whole setup around it. Thus, sending IRlight to the sample could be done only through infrared transparent viewports that,of course, had to be located accordingly to the angle of incidence adopted for RAIRS.There were two main constraints that could not be eluded:a) the position of the sample was not to change, namely, located at center of themain chamber, and at the same level of the beamlines. This was motivated by thedesire to maintain the big advantage of having the two atomic/molecular beamlinesstill aimed at sample and also for avoiding modifications of the overall sample holderdesign.b) the configuration of the main chamber ports through which the entering andexiting IR beam had to pass. In fact, as shown in Fig. 4.10, there were a fixed numberof ports whose locations could not be changed, unless taking down the main chamber(i.e., the whole apparatus!) for long and complex machining work.In Fig. 4.10 is shown the possible choice of two already present ports (port Aand E) that would give the highest angle of incidence. In this configurations no big

82 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.10: Port configuration the of main chamber (top view). With almost no modificationsof the present apparatus, the highest angle of incidence (67 ◦ ) onto the sample canbe obtained between port A and port E.modifications of the apparatus were required, except the substitution of the 8 inchviewport with a custom flange suitable to accept an IR window. Unfortunately, thisconfiguration gives the possibility of an incident angle of ∼ 67 ◦ at the most respectto the sample normal. We have seen earlier that the incident angle for maximumsensitivity lies between 85 ◦ and 89 ◦ . Some authors actually use smaller grazing angles,but no one uses incident angles smaller than 75 ◦ .Therefore, I have discarded this possibility of employing an angle of 67 ◦ , in partbecause no other author used such a relatively low angle of incidence, and also becausethe relative sensitivity at 67 ◦ is only 15% of the maximum found at ∼ 86 ◦ (Hollins1994). Hence, an incidence angle of 67 ◦ could turn out to be too small for a successfulapplication of RAIRS. Finally, the use of ports A and E would lead to the inconvenient

4.4. Importing FT-RAIRS into our laboratory 83Figure 4.11: Proposed configuration to achieve a possible incident angle between 76 ◦ and90 ◦ . This configuration makes necessary to move the TOF appendix.experimental condition of not having the sample facing the beamlines.After these preliminary considerations, I decided to propose an alternative configuration(see Fig. 4.11) in which either the incident or the reflected IR beam wentthrough the 8 inch port where the TOF appendix was mounted. The disadvantage ofsuch configuration entailed the dislocation of the entire TOF appendix to a differentavailable port. However, this arrangement would allow us to perform RAIRS studiesat an incidence angle chosen between 76 ◦ and, in theory, 90 ◦ .Key references in this preliminary study and in the subsequent design were Hollins(1994), Trenary (2000), Fraser et al. (2002a), Bennett et al. (2004).

84 Chapter 4. Construction of the FT-RAIRS facility4.4.2 The choice of the 78 ◦ incident angle and overview ofthe new configuration proposalAfter the preliminary study, it was clear that our apparatus was “eligible” to be integratedwith a FT-RAIRS facility, at the expense of some modifications, i.e., mountingthe TOF appendix to another available port, and adapting the 8 inch ports for mountingIR windows.We then decided to begin the construction of a RAIRS configuration which wasto employ an incident angle of 78 ◦ . This choice was motivated by certain constraintsconcerning the space around the main chamber and the design of the new customflanges, as well as geometric considerations regarding the sample holder and the angularspread of the incoming IR beam that was to impinge on a sample surface lessthan one cm 2 .In Fig. 4.12 is shown the new configuration which was chosen to perform RAIRStechnique in our laboratory. The top view presents the main chamber, the FT-IRspectrometer, and the two enclosure #1 and #2 custom-designed to house the optics.The FT-IR spectrometer will have its external collimated IR beam entering enclosure#1. This enclosure houses the optics that will steer the IR beam onto the samplein the main chamber through a IR transparent window (e.g., a potassium bromidecrystal) mounted on the flange between enclosure #1 and the main chamber. Thefirst mirror will be a flat ellipsoidal (gold coated) mirror used to reflect the collimatedIR beam from the spectrometer to the off-axis paraboloidal mirror, which then willfocus the beam onto the sample. The off-axis parabolic mirror will be gold coatedand have an effective focal length of ∼ 14 inches. Through a second IR window, thediverging IR beam reflected off the gold coated surface of the sample substrate willbe refocused onto the detector via an off-axis ellipsoidal gold coated mirror. Boththe detector and the second focusing mirror will be housed within enclosure #2. TheFT-IR spectrometer and the two enclosures will be purged either with nitrogen ordried air to eliminate from the IR path the strong absorption bands caused by gascontaminants, such as H 2 O and CO 2 , in the mid-IR range.The choice of this optical arrangement was dictated by the fact to have the lowestnumber of mirrors along the optical path followed by the IR beam. Since a certainamount of light is always lost in a reflection, due to the imperfect reflectivity ofmirrors, and also because sensitivity is one limitation of RAIRS, I reduced to theminimum the number of mirrors in order to get more light to the detector. A reviewof Bradshaw & Schweizer (1988) provides a detailed analysis of ways to optimise theoptical configuration of FT-RAIRS.The UHV main chamber will also be equipped, specifically for this configuration,with two custom “8 to 3 3/8 inch” flanges (described below) and two differentiallypumped 3 3/8 inch flanges on which IR windows will be mounted to allow IR light to

4.4. Importing FT-RAIRS into our laboratory 85Figure 4.12: Proposed FT-RAIRS scheme with an incident angle of 78 ◦ . F, P, and E arethe flat, off-axis parabolic, and off-axis ellipsoidal mirrors respectively. D is the detector.enter/exit the main chamber with the desired incidence angle.Finally, the described RAIRS setup was designed to operate in the spectral range5000 - 500 cm −1 (2 - 20 µm), thus covering the most important vibrational spectralfeatures of many astrophysically significant ices (e.g., H 2 O, CO, and CO 2 ) andmolecules (e.g., H 2 CO, and CH 3 OH).In the following sections I shall take up in details the design and the motivationfor the use of each piece that makes up the new RAIRS facility.

86 Chapter 4. Construction of the FT-RAIRS facility4.5 The Nicolet TM FT-IR 6700 SpectrometerThis section describes the major components and characteristics of the FT-IR spectrometerthat we purchased for our new FT-RAIRS facility.We decided to employ the FT-IR spectroscopy in our laboratory, because the useof commercial FT-IR spectrometers has the advantage that the huge analytical instrumentsmarket, and the associated competition among manufactures, has led to bigadvances in instrument performances while keeping costs relatively low. Besides, thesame general advantages that an FT-IR spectrometer offers over a grating instrumentapplies to the RAIRS experiments and are even greater when high resolution over awide range of wavelengths is desired.One of our most important requirements for the FT-IR spectrometer to be usedin such a custom arrangement was the possibility to employ it by using an externalcollimated IR beam from the FT-IR that we would steer to the sample, and then toan external detector as well. Other important requirements were the spectral rangeof operation, which had to include the mid-IR wavelengths at least (4000-400 cm −1 ),and a high resolution, of the order of 1 cm −1 .Eventually, we decided to purchase the Thermo Electron Nicolet TM FT-IR 6700.Thermo Electron is known to be a very good FT-IR manufacturer, and also manyother researchers use their products with excellent results. In Fig. 4.13 is shown atop view of the FT-IR 6700 spectrometer where its most important components andthe optical layout are visible. The FT-IR 6700, in its standard version, comes witha potassium bromide (KBr) beamsplitter, which makes it suitable for spectroscopicstudies between 7800 and 400 cm −1 , and gold coated optics. Table 4.1 reports themain characteristics of the FT-IR 6700.As most FT-IR spectrometers today, Nicolet FT-IR 6700 is completely computerizedand can be operated totally through a proprietary software called OMNIC TM ,written for Nicolet FT-IRs. OMNIC provides a powerful Windows R compatible interfacefor complete data collection and processing, data displays, active spectrometerdiagnostics, and spectral quality checking.4.5.1 IR sourceThe infrared light source makes a significant contribution to the performance of anIR spectrometer, since the signal-to-noise ratio of the instrument is directly relatedto the source energy.On the Nicolet FT-IR 6700, the standard light source is a high-intensity ETCEverGlo mid-IR source. The ETC EverGlo source is an efficient ceramic, refractorycomposite that rapidly rises to operating temperature and is also thermally insulatedto maintain a constant operating temperature over the 7400 - 50 cm −1 spectral region.One of the main features of this model is that it is capable of operating in three

4.6. The MCT detector 87Figure 4.13: Top view of the optical layout of the FT-IR 6700 spectrometer with coversremoved to show its standard and optional components.distinct modes: Normal, Turbo, and Rest. In the Normal operating mode, the sourcetemperature is constantly monitored and controlled at 1140 ◦ C by the ETC EverGlocircuit. The Turbo mode enables the user to increase the temperature of the sourceto 1250 ◦ C. There is an increase in intensity of 25 - 45% from 2200 to 4000 cm −1 ,as shown in Fig. 4.14. Finally, the Rest Mode allows the source temperature to belowered during periods of inactivity and thus prolongs the life and improves the longterm output of the source. Rest Mode activates when the spectrometer has beeninactive for a period of time pre-specified by the user, lowering the temperature ofthe source to 900 ◦ C.4.6 The MCT detectorFor our application of RAIRS, we use an external IR detector. Among the other systemcomponents, the detector is particularly important since it has to match uniquelyto the specific application. Thus, the choice of the ideal detector for spectral measurementis dependent upon many factors, including:a) Optical throughput (percent of beam reaching the detector)b) Spectral range of the measurementc) Temporal resolution of the data collectiond) Spectral resolution.High throughput (greater than 20% of IR beam reaching the IR detector) andstatic experiments are generally run utilising a thermal detector, such as deuterated

88 Chapter 4. Construction of the FT-RAIRS facilityTable 4.1: Main characteristics of the Nicolet FT-IR 6700 spectrometer.Spectral Range (KBr beamsplitter)7800 - 350 cm −1Gold-Coated OpticsYesOptical Resolution0.09 cm −1Peak-To-Peak Noise (1 minute scan)< 8.68 x 10 −6 AU ∗∗RMS Noise (1 minute scan)< 1.95 x 10 −6 AU ∗∗Ordinate Linearity0.07 % TWavenumber Precision0.01 cm −1Slowest Linear Scan Velocity0.158 cm/secFastest Linear Scan Velocity6.33 cm/secNumber of Scan Velocities 15Rapid Scan (Spectra/second 16 cm −1 , 32 cm −1 ) 65, 95Throughput Increase in Turbo Mode ( 3000 cm −1 ) 25%Liquid Nitrogen-Cooled Detector Hold Time18 hours∗∗ Absorbance UnitsL-alanine doped triglycine sulfate (DLaTGS), because it gives full specific detectivity 1in high-flux experiments. This means that they provide linear response over a verywide range of FT-IR throughput.In our case, though, owing to the number of reflections of the IR beam and a definiteimperfect reflection of the sample substrate, RAIRS applications are expected toresult in low-throughput experiments. This led us to purchase a quantum detector,namely, a liquid N 2 cooled Mercury Cadmium Telluride (MCT) detector. In fact,high MCT sensitivity will produce a large signal in a low-flux measurement. Furthermore,the MCT detector demonstrates a relatively constant signal versus datacollectionspeed and is, therefore, ideal for kinetic measurements. A limitation ofMCT detectors is that they lose responsivity and D ∗ at high throughput. From theapplications perspective, this means that in high-throughput applications, we mustlimit the beam energy reaching the MCT detector to prevent saturation, wheneverthe detector responds with a non-linear signal. Limiting the beam energy can be doneby using neutral density filters, or optical screens, while reducing the beam intensityby narrowing the beam diameter is not effective. Nonetheless, in RAIRS experiments,because the adsorbate vibrations interact only with the p-polarised component of theradiation, the use of a polariser also restricts the amount of radiation reaching thedetector.1 The specific detectivity (indicated with D ∗ ) is a measure of the detector signal as a functionof energy flux and detector noise. At low-light levels, the D ∗ number may be directly comparedfrom one detector type to another. For example, the MCT-A ∗ detector with a D ∗ of 6.4 × 10 10 isapproximately 237 times more sensitive than the DLaTGS detector with a D ∗ of 2.7 × 10 8 . It isimportant to consider that detectors with high D ∗ values tend to demonstrate saturation effects inhigh-throughput experiments.

4.7. The custom 8 - 3 3/8 inch coupling flanges and the DPWs 89Figure 4.14: Single beam intensity comparison of Turbo and Normal modes of the infraredECT source.In our system, we shall be using the detector type MCT-A manufactured byThermo Electron, whose specifications are reported in Table 4.2. The MCT-A detectorwill be located within enclosure #2. I shall discuss below that the positioningof the detector has to be very precise since its sensor element will have to be lying onone of the conjugate foci of the off-axis ellipsoidal mirror. The height of the detectorin respect to the enclosure base can be adjusted (from the outside of the enclosurebase) by three threaded rods located beneath the detector base plate.Table 4.2: Characteristics comparison of the most common detectors used in in the mid-IRmanufactured by Thermo Electron.Detector Element Spectral Typical D ∗ MinimumElement/Window Size Range (cm Hz 1/2 W −1 ) ResponsivityDLaTGS/KBr ∅ 1.3 mm 12500-350 cm −1 2.7 × 10 8 50 V/WDLaTGS/CsI ∅ 1.3 mm 6400-200 cm −1 2.4 × 10 8 50 V/WMCT-A*/CdTe 1.0 × 1.0 mm 2 11700-800 cm −1 6.4 × 10 10 1,200 V/WMCT-A/CdTe 1.0 × 1.0 mm 2 11700-600 cm −1 4.7 × 10 10 750 V/WMCT-B/CdTe 1.0 × 1.0 mm 2 11700-400 cm −1 8.0 × 10 9 50 V/W4.7 The custom 8 - 3 3/8 inch coupling flanges andthe DPWsIn order to couple the external enclosures housing the optics and the MCT detectorto the UHV chamber there was the need to design and build two custom flanges. InFig. 4.12 we have seen that the ports through which we want the IR beam to enter

90 Chapter 4. Construction of the FT-RAIRS facilityand exit the main chamber are the two big opposite 8 inch ports. Thus, to couplethe main chamber ports to the purged enclosures, I designed two identical customflanges whose arrangement is shown schematically in Fig 4.15. Each coupling flange,machined in the machine shop of the Physics Dept. at SU, was made out of an 8inch and a 3 3/8 inch Conflat R flanges attached by a stainless steel tube. The 3 3/8flange central axis forms an angle of 12 ◦ in respect to the 8 inch flange axis. This wasdesigned to have the IR beam enter the main chamber and aim at the sample withthe desired incidence angle of 78 ◦ .Figure 4.15: Dimensions and arrangement of the custom-made “8 to 3 3/8 inch” flange(bold line) mounted on the main chamber (dashed line).The interface between the UHV main chamber and the enclosures (at atmosphericpressure) is an IR transparent crystal, such as KBr or KCl, which is commonly veryfragile and can be mounted by using only O-ring gaskets. In fact, most IR transparentmaterials break easily and are hygroscopic as well. They tend to absorb water vaporcontained in the air, eventually fog and become opaque. In Fig. 4.16 are reported themost common window materials used in the mid-IR.Although there are commercially available IR viewports of various sizes mountedon Conflat flanges, they are not suitable for UHV application, and also they tend tobe very expensive. For this reason, I decided to use a couple of Differentially PumpedWindow (DPW) flanges (one for each IR window) which are used to mount specialwindows to the ultrahigh vacuum systems. In fact, by pumping on the differential

4.8. The custom-made off-axis mirrors 91Figure 4.16: Wavelength range of transparency for IR materials, in µm (top), and cm −1(bottom).section with simply a mechanical pump, UHV pressures on the inside can be routinelyachieved. These type of window flanges allow the user to easily remove, polish andreplace windows that have become opaque, thereby extending their useful life. Also,if either the experimental requirements change or the window crystal fogs or breaks,only the window itself needs be changed, at considerable cost savings.The differentially pumped windows used in our apparatus are two 3 3/8 inch DPWflanges manufactured by McAllister, Idaho (USA). See Fig. 4.17.4.8 The custom-made off-axis mirrorsMost FT-RAIRS configurations use Off-Axis Parabolic (OAP) mirrors for collimatingand focusing light external to the spectrometer.Off-axis parabolic reflectors are a circular segment from one side of a full paraboloid.The focal point is off the segment axis, giving full access to the reflector focus area.Unlike standard parabolic mirrors, these mirrors direct and focus incident collimatedlight at a specific angle (for example 30 ◦ , 60 ◦ or 90 ◦ ), allowing unrestricted access tothe focal point. The arrangement shown in Fig. 4.18 represents a 90 ◦ off-axis parabolicmirror since the ray striking the center of the aperture and parallel to the main axisturns exactly at 90 ◦ and comes into the focal point. The distance from the point onthe surface of the parabola at the center of the aperture, to the focal point, is calledEffective Focal Length (EFL) and in a 90 ◦ OAP configuration , by construction, it isexactly two times the Focal Length (FL) of the parent paraboloid.Similarly, off-axis ellipsoidal mirrors are used to redirect light maintaining fullaccess to the two focal points. In fact, ellipsoid reflectors have two conjugate foci, so

92 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.17: Schematic blow out of a 3 3/8 DPW flange by McAllister.light from one focus passes through the other, after reflection.In the case of gold coating, mirrors are very broadband, from 0.7 to 10 µm theyreflect more than 98% of the incident radiation, and this percentage stays in thisrange up to 25 µm. Copper and silver are also two highly reflective metal surfacesused commonly for applications in the mid-IR spectral range, with a reflectance percentagecomparable to that of gold. Also, an important feature of mirrors in general,unlike lenses, is that they do not have any dispersion; there is no chromatic aberrationso the focal spot stays at the same place for any wavelength.For the FT-RAIRS setup to be employed in our laboratory, I designed a 90 ◦ off-axisparabolic mirror. This mirror will serve to focus the collimated IR beam coming fromthe FT-IR spectrometer, after a reflection on a planar gold coated mirror, onto thesample at the center of the UHV chamber. The diameter of the mirror is 2.65 inches,and has an effective focal length of 14 inches. Although various types of OAP mirrorsare commercially available, I had to custom-design it because of the relatively longEFL required in our configuration. A technical drawing shown in Fig. 4.19 reportsthe main specifications of the 90 ◦ OAP mirror designed for our arrangement. I alsodesigned an off-axis ellipsoidal mirror in order to use only one optical piece to refocusthe diverging IR beam reflected from the gold coated sample substrate onto the detector.This OAE mirror was custom-designed as well from an ellipsoid with a 8.86 inchsemi-major axis, and a 4.42 inch semi-minor axis. A technical drawing of the OAE

4.9. The final FT-RAIRS setup 93Figure 4.18: Schematic of a 90 ◦ off-axis parabolic configuration.mirror is given in Fig. 4.20.Both the OAP and the OAE mirror are electroformed nickel custom reflectorsmanufactured and gold coated by Opti-Forms Inc., California. These type of mirrorsare nickel-grown replications of the surface of highly polished stainless steel toolingmandrels that have a surface accuracy of about 6 to 8 λ. These masters are thencomputer-machined as accurate as possible to achieve a surface accuracy of a few λ.Thus, these mirrors provide a highly accurate illumination and control of light energy,although they are not suitable for highly demanding imaging applications. The manufactureralso states that the surface quality of the mirrors is 80/50 (scratch/dig),thus a very good quality for mid-IR applications.The two off-axis mirrors (and the flat mirror too), are mounted on adjustablekinematic mirror mounts by Edmund Optics Inc. These kinematic mounts allowadjustment via the movement of two fine 80-pitch screws, and permit 7 ◦ tilt movementin two planes (7 ◦ in X-Axis, 7 ◦ in Y-Axis) for precise position of mounted optic.4.9 The final FT-RAIRS setupThe final configuration of the FT-RAIRS setup built in our laboratory is shown inFig. 4.21 (spectrometer side) and in Fig. 4.22 (detector side). The external 1.5 inchdiameter beam from the FT-IR 6700 enters enclosure #1 and is first steered usingone flat gold coated elliptical mirror (60/40 surface quality), then deflected again by90 ◦ and focused onto the sample with an incident angle of 78 ◦ through a differentiallypumped KBr window. The focusing mirror is a 90 ◦ off-axis parabolic mirror with aneffective focal length of 14 inches. A top view drawing of enclosure #1, with all theoptics in place, is given in Fig. 4.23. The sample substrate is made of a solid OFHC Cublock coated in thin film of poly-crystalline gold, thus acts as a flat reflective mirror.

94 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.19:mirror.Technical drawing and dimensions of the custom off-axis 90 ◦ parabolicThe reflected IR beam, through a second differentially pumped KBr window entersenclosure #2 and is then focused onto the liquid N 2 cooled MCT detector element(1 mm 2 ). The second focusing mirror is an off-axis ellipsoidal reflector that, at onceand without using any other a piece of optic, collects the diverging IR beam arrivingfrom the sample to refocus it on the detector element. Much care was taken to setthe position of the detector within enclosure #2, since it was crucial that the detectorelement lied on one of the two conjugate foci (denoted by F1) of the ellipsoidal mirrorto collect as much of light as possibly feasible from the other focus (F2), lying exactlyat the center of the sample. See Fig. 4.22 and Fig. 4.24 for a graphical reference. Theoutput signal from the detector is fed back to the FT-IR 6700.The FT-IR spectrometer and the two enclosures that house the entire optics arepurged with dry-air (continually dried at a dew point of -40 ◦ C/ ◦ F, or at a dew pointof -73 ◦ C/-100 ◦ F during experiments) provided by a dual tower desiccant dryer. Thedryer is fed with house compressed air at ∼ 80 psi (∼ 5.4 atm). The dried air is sentto the spectrometer and the enclosures through a teflon tube (1/4 inch diameter).

4.9. The final FT-RAIRS setup 95Figure 4.20: Technical drawing and dimensions of the custom off-axis ellipsoidal mirror.The dried air flow is set to produce a slight overpressure to remove IR active species(especially water vapor) from the beam path and also to preserve the KBr beamsplitterinside the FT-IR 6700 and the two KBr windows.The lids of the enclosures can be removed for alignment purposes. Finally, O-ringswere placed between the spectrometer and enclosure #1, under the enclosures lids,and between the main chamber flanges and enclosures, although no air-tight sealswere used.

96 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.21: Final FT-RAIRS setup (spectrometer side).

4.9. The final FT-RAIRS setup 97Figure 4.22: Final FT-RAIRS setup (detector side).

98 Chapter 4. Construction of the FT-RAIRS facilityFigure 4.23: Detailed drawing of enclosure #1 with the planar and OAP optics in place.

4.9. The final FT-RAIRS setup 99Figure 4.24: Detailed drawing of enclosure #2 with the OAE optic and the MCT detectorin place.

100 Chapter 4. Construction of the FT-RAIRS facility

Chapter 5Summary and conclusions5.1 Astrochemistry - Molecules in the ISMAstrochemistry is today a firmly established subject, describing the pathways by whichthe molecules are formed and destroyed in space, and in those processes identifyingnew fundamental chemistry. The great deal of interest for molecular species stemsfrom the crucial role that molecules play in the Universe. They act as excellent probesof the physical properties and dynamics which characterise the astronomical regionsin which they are located. Molecules also play an active role in the energy balance ofinterstellar clouds, affecting the rate of ionisation and the thermal balance, and thusinhibiting or triggering dynamical instabilities. Indeed, through molecules we manageto understand how galaxies and stars are formed, how new stars interact with theirenvironments, and how old stars evolve and die.So far, the inventory of interstellar and solar system molecules now numbers around130 molecular species, although interstellar chemistry implies the existence of at leastseveral hundreds of molecules. This entails a great many more species that have not sofar been detected. However, the most abundant molecule is by far molecular hydrogen(H 2 ). Molecular hydrogen, besides playing the most important role as a naturaltemperature regulator, is also crucial because it intervenes, either in its charged orneutral form, in virtually all reactions schemes leading to the formation of bigger andmore complex molecules in the ISM.Furthermore, complex molecules, such as ammino acids, are the fundamental“units” that make up body proteins, which are the building blocks of life on earth.Searching for complex molecules like ammino acids, or sub-structures of ammino acidsin the interstellar medium may turn out to be important to understand how life onearth, or elsewhere, evolved.

102 Chapter 5. Summary and conclusions5.2 Grain-surface reactions and H 2The dust grains prove to be important components of the interstellar medium, andare now recognized as playing a fundamental role in the evolution and current state ofthe Universe. The most obvious and apparent effect of dust grains is the extinction ofstarlight. This also means that in regions of space that are shielded from starlight, thephotodissociation of molecules is hindered, and thus a rich chemistry becomes possible.Besides, dust grains provide sites (their surface) for many chemical reactions. In fact,it is now recognised that gas-phase chemistry alone cannot explain the high observedhigh abundances of certain molecules. Among the various species whose formationhas to be catalysed by the presence of solid surfaces in clouds molecular hydrogen iscertainly the foremost. H 2 is by several orders of magnitude the most abundant speciesin the Universe, and it is by far the most important in interstellar clouds because it isinvolved in all ion-molecule reactions schemes relevant to gas-phase synthesis of almostall the remaining molecules observed. Furthermore, H 2 molecules, once collisionallyexited to metastable energy states, relax by radiating infrared photons that easilyescape from the clouds; thus H 2 stands out as a very efficient coolant that increasesthe rate of collapse of interstellar clouds contributing to shape galaxies and to drivetheir dynamics.The process of molecular hydrogen formation must be very efficient to form theobserved large amounts of H 2 in diffuse clouds, as it is also continually destroyed byultraviolet light. Thus, since H 2 cannot form efficiently in the gas phase, the widelyaccepted mechanism for the formation of H 2 in the ISM is that it forms through acatalytic reaction between hydrogen atoms on the surface of cosmic dust grains, wherethe grains act as a third body in the H + H reaction.5.3 Experimental apparatus at SUAmong the most interesting results accomplished in astrophysics, we find those achievedby combining observational and theoretical data with the outcome of laboratory work.In astrochemistry, laboratory experiments help us to identify molecules relyingon precise laboratory spectra, or simulate solid-phase molecular syntheses in orderto reconcile the predicted and the observed abundances by providing models withdefinite constraints.During my doctorate work, I carried out an experimental investigation of H 2 formationon diverse samples of amorphous olivines. The experiments were carried outthrough mass spectrometry and TPD techniques and under conditions that comeas close as technically feasible to the ones in the most relevant ISM environments,namely, under ultra high vacuum pressures and at surface temperatures between 6and 30 K. The apparatus is located in the Physics Department of Syracuse University

5.3. Experimental apparatus at SU 103(N.Y., USA), and consists of two atomic/molecular beam lines, one UHV chamberwhere the sample and detector are located, and a time-of-flight section, see Fig. 5.1.Each atomic/molecular beam line has three differentially pumped stages. The firstFigure 5.1: Schematic of the apparatus (top view). S1 and S2: H 2 and D 2 RF dissociationsources; CH1 and CH2: mechanical choppers; T1, T2, and T3: turbo molecular pumps; Ti:titanium sublimation pump; C: cryopump. Adapted from Roser et al. (2002).stage houses a water-cooled Pyrex nozzle source that is surrounded by an RF cavityused for dissociating hydrogen or deuterium. The use of two beam-lines proved to beessential in experiments of molecular hydrogen formation, since we have the possibilityto use one atomic H beam and one atomic D beam simultaneously and observethe formation of HD with a high S/N ratio. Estimated beam fluxes at the sample areof the order of 10 12 atoms cm −2 sec −1 .The two beams enter the main UHV scattering chamber via two gate valves andare carefully aimed at the same spot on the sample. This chamber houses a rotatabledifferentially pumped quadrupole mass spectrometer that is used for measuring theintensity of the incoming beams and the products of reaction evolving from the surfaceof the sample. The operating pressure in the main chamber is in the 10 −10 torr range.The sample is mounted on a UHV compatible cold finger bolted onto a rotatableflange. Prior to cooling, the sample can be heated to 400 K for cleaning. After cooling,the temperature of the sample can be adjusted by throttling the liquid helium flow tothe cryostat and by using a resistive heater located in a ceramic box behind the sample.The lowest temperature to which the sample can be cooled is ∼ 4.5 K, as measuredby a calibrated silicon diode thermometer located behind the sample. Furthermore,the low temperature stage is surrounded by a radiation shield thermally anchored to astage of the cryostat. This arrangement prevents the sticking of particles of the beamon other parts of the sample holder.

104 Chapter 5. Summary and conclusions5.4 H 2 formation on amorphous olivineThe experimental work presented in this thesis was conducted in Syracuse as part ofthe most successful programme of experiments so far to study the processes involvedin the formation of molecular hydrogen on a variety of dust analogue materials.The experiments described here were planned to investigate hydrogen recombinationon a series of amorphous olivine (a type of silicate) surfaces. In fact, olivinesrepresent one of the most realistic dust grain analogues relevant to diffuse environment.In our experiments we used samples of amorphous olivines of type (Mg x ,Fe 1−x ) 2 SiO 4 ,with a relative fraction x of Mg, in respect to Fe, being 1, 0.75, 0.50, 0.25, and 0.We found that HD desorption traces given by the amorphous surface are farbroader than that given by the poly-crystalline olivine (Pirronello et al. 1997a,b).Also, it is apparent that the maximum of the amorphous olivine is at a higher temperature.From this fact we can infer that a broader distribution of sites and a higheractivation energy barrier exist for amorphous materials.The recombination efficiency obtained experimentally is Sγ ∼ 0.2 between 10 and20 K for most of the amorphous olivine samples, and ∼ 0.4 - 0.5 for a couple ofsamples that showed a higher value of Sγ. This established the actual feasibility of H 2formation on cosmic dust analogue surfaces in simulated astrophysical environmentswithin a wider range of temperatures (15 - 30 K) than previously found. The samplesto which was attributed a higher recombination efficiency seemed to have a morecomplex surface structure, and also these roughest samples showed a wider rangeof temperatures over which the catalytic process occurred. Therefore, we concludedthat the morphology of the sample surface plays an important role in the formationof molecular hydrogen.In Fig. 5.2 are reported recombination efficiencies obtained for all classes of dustgrain analogues studied so far. The fact that we obtained similar results for allmaterials indicates that the process that dominate adsorption and diffusion of Hatoms is due to weak physical adsorption interactions.Finally, experimental evidence, such as the observed second-order kinetics in theprocess of HD recombination and the difference between TPD experiments done withdoses of H+D and HD molecules, suggest that the mobility of H and D ad-atoms necessaryto an effective diffusion on the surface to form HD is given by either thermalhopping or by what can be called “thermally assisted tunneling”, namely, the heatpulse provided during TPDs would favor tunnelling among physorption sites. Thus,it is the Langmuir-Hinshelwood mechanism that drives the process of molecular hydrogenrecombination in the coverage regime reproduced in our laboratory, and, inturn, in diffuse clouds, where much lower coverages characterise dust grain surfaces.

5.5. Construction of the FT-RAIRS facility 105Recombination Efficiency0,70,60,50,40,30,20,10,0Poly-crystalline OlivineAmorhous CarbonHigh Density H 2O iceLow Density H 2O ice (heated)Low Density H 2O ice (vapor-deposired)Amorphous Olivine (high eff. group)(batch #1, sample #5)Amorphous Olivine (low eff. group)(batch #1, sample #4)6 8 10 12 14 16 18 20 22 24 26 28Temperature at Irradiation (K)Figure 5.2: H 2 recombination efficiency for all major classes of dust grain analoguesstudied at SU. Amorphous olivine samples (filled red signs) studied in this work.5.5 Construction of the FT-RAIRS facilityBesides molecular hydrogen, many other molecular species are believed to be producedvia reactions occurring on dust grain surfaces. In fact, species such as H 2 O,CO 2 , H 2 CO, and CH 3 OH are believed to form by grain-surface reactions throughhydrogenation and oxidation reactions. Therefore, in order to extend experimentalinvestigations to more complex molecules than H 2 , our group decided to integratethe experimental apparatus with a new piece of equipment to perform surface infraredspectroscopy, namely by the construction of a facility to employ the techniquecalled Fourier Tranform Reflection-Absorption Infrared Spectroscopy (FT-RAIRS).The choice of RAIRS stemmed from the advantage of being able to investigate adsorbedmolecules on a bulk metal. This technique consists of reflecting a beam ofinfrared radiation off a metal surface (through an adsorbed layer) and looking at theloss in intensity of the reflected light at frequencies which correspond to the vibrationalmodes of either the adsorbed species itself or as a result of the interaction ofthe adsorbed species with surface atoms of the substrate.The arrangement of the custom-made FT-RAIRS facility is shown in Fig. 5.3.The external IR beam from the FT-IR spectrometer enters enclosure #1 and is firststeered using one flat elliptical mirror, then deflected again by 90 ◦ and focused ontothe sample through a differentially pumped KBr window with an incident angle of78 ◦ . The focusing mirror is a 90 ◦ off-axis parabolic mirror with an effective focallength of 14 inches. The sample substrate is coated in thin film of poly-crystallinegold, thus acts as a flat reflective mirror. The reflected IR beam, through a seconddifferentially pumped KBr window enters enclosure #2 and is then focused onto theliquid N 2 cooled MCT detector element. The second focusing mirror is an off-axisellipsoidal reflector that collects the diverging IR beam arriving from the sample to

106 Chapter 5. Summary and conclusionsrefocus it onto the detector element.As set out in Table 5.1, many are the molecular species identified by their infraredabsorption features and observed on dust grains in form of ices. FT-RAIRS will allowus to analyse not only a much wider number of adsorbed species, but also monitorand track changes of the adsorbates during the experiments. Also, the combinationof mass spectrometry, TPD, and FT-RAIRS techniques will be key to develop ourlaboratory-based research.Figure 5.3: Scheme of FT-RAIRS setup with an incident angle of 78 ◦ , custom-made inour laboratory. F, P, and E are the flat, off-axis parabolic, and off-axis ellipsoidal goldcoated mirrors respectively. D is the MCT detector. The FT-IR spectrometer and bothenclosures housing the optics can be purged either by N 2 or dried air.5.6 ConclusionsThe results achieved in this work give an example of how surface science experimentscan contribute to the field of astrochemistry, by providing reliable informations onfundamental processes that occur on bare surfaces of cosmic dust grains. The recombinationefficiency of molecular hydrogen under conditions close to the interstellar

5.7. Future projects 107Table 5.1: Summary of interstellar molecules identified in form of ices by their infraredabsoprtion features (Boogert & Ehrenfreund 2004).Molecule Infrared bands (µm)H 2 O 2.96, 3.07, 3.2-3.7, 4.5, 6.0, 12, 44CO 2 / 13 CO 2 2.70, 2.78, 4.27, 15.2 / 4.38CO/ 13 CO 4.67 / 4.78OCS 4.92H 2 CO 5.83HCOOH 5.83, 7.25CH 4 3.32, 7.67CH 3 OH 2.27, 3.54, 3.85, 3.94, 4.1, 6.85, 8.9, 9.7SO 2 7.6NH 3 2.96, 3.2 - 3.7, 3.47, 9.01HCOO − 7.41OCN − 4.62PAH, C-C 6.25, 7.60medium was measured on low temperature surfaces of amorphous olivines. The desorptionkinetics of the process was analysed as well. We found that the Langmuir-Hinshelwood is at work in a low coverage regime. These new results upon hydrogenrecombination in space are of particular interest because they were carried out onone of the most realistic materials believed to make up dust grains surfaces in thediffuse interstellar environment. Moreover, the role of morphology was underlined.H 2 formation on amorphous olivine proved to be significantly more efficient than onthe poly-crystalline material, and the catalytic process on the amorphous surface wasobserved to occur over a wider range of temperatures.Also, the desire to extend the past and the newest remarkable results achieved byour group to more complex molecules than the key species H 2 , led us to the constructionof a custom-made FT-RAIRS facility in order to have TPD experiments assistedby a powerful tool of surface IR spectroscopy. FT-RAIRS will allow us enhance ourlaboratory investigations and conduct the next experiments with more detail andaccuracy.5.7 Future projectsOne of the next challenges for experimental astrochemists is the extension of studiesof molecular hydrogen formation to higher temperature regimes. In fact, likewisein diffuse interstellar clouds, observed abundances of molecular hydrogen in PDRscan only be explained by heterogeneous catalytic reactions occurring on dust grainsurfaces. The temperature of grains in PDRs ranges between 15 and 100 K, while gas

108 Chapter 5. Summary and conclusionstemperatures are in the range 300 - 1000 K, and also a strong UV radiation field ispresent. These ambient conditions thus make H 2 formation on dust grain feasible onlyif chemisorption sites are involved in the adsorption of H atoms on dust grain surfaces.Next experiments will have the purpose to investigate whether molecular hydrogenrecombination can really occur on cosmic dust analogues at temperatures in the range30 - 100 K, and possibly to which mechanism, Eley-Rideal or Langmuir-Hinshelwood,this key reaction can be ascribed.Another main project is related to measurements of the formation rate of watermolecules on surfaces of amorphous carbon or olivine, and on surfaces of low- andhigh-density amorphous ice. Water ice is the main component of icy mantles thatform on dust grains in denser clouds. It is widely recognised that mantle growthcannot occur by direct accretion of water molecules from the gas phase, but thatgrain-surface reactions among accreting atoms, radicals, and molecules could betterexplain H 2 O ice observations by ISO. In particular, hydrogenation of oxygen atomswould produce large amounts of water molecules right on grain surfaces. Yet thiscrucial reaction has not been proven by experiments and our group, now enrichedwith the possibility to perform surface infrared spectroscopy by RAIRS, considers itas a major project for the next future.

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