Overtone spectroscopy of H2O clusters in the vOH - UCI Aerosol ...

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194316-2 Nizkorodov et al. J. Chem. Phys. 122, 194316 2005FIG. 1. Experimental approach. Complexes are excited in v OH =2 state followedby photodissociation of H 2 Ov OH =2 directly inside the complexeswith a UV laser pulse left. Alternatively, the excited complexes first predissociateon time scale pd generating H 2 O molecules in a different vibrationalstate v, which are then photodissociated by the photolysis laserright. In either case, the resulting OH fragments are detected in specificfinal quantum states by laser-induced fluorescence.resolution and provides information about the dynamics ofH 2 O 2 predissociation at the v OH =2 excitation energies7000 cm −1 .By way of contrast, the much simpler complex betweenAr and H 2 O provides a useful juxtaposition with H 2 O 2 . 40,41Compared to the water dimer, Ar–H 2 O has a substantiallysmaller potential energy well depth 140 cm −1 in Ar–H 2 Ovs1700 cm −1 in H 2 O 2 Refs. 18 and 42 and considerablyweaker interactions between intermolecular and intramolecularmodes. This makes Ar–H 2 O a convenient system forstudying photodissociation dynamics of H 2 O in the presenceof a weakly perturbing rare gas “solvent” as opposed to thestrongly hydrogen-bonded interactions present in H 2 O 2 .Vibrationallymediated dissociation studies of Ar–H 2 O in thev OH =3 manifold have been reported 40 but none in the firstovertone region corresponding to the present study of H 2 Odimer. To establish a suitable experimental perspective forthe more complicated spectra of H 2 O 2 , this study thereforealso briefly considers vibrationally mediated spectroscopyand dynamics out of selected v OH =2 vibrational states ofAr–H 2 O.II. EXPERIMENTPertinent experimental information has been summarizedin recent work dealing with the dynamics of vibrationallymediated dissociation of H 2 O monomer in the v OH=2 polyad; 43 thus only the most relevant details are summarizedhere. Ar–H 2 O and H 2 O 2 complexes are produced ina supersonic expansion of 1% H 2 O in 30% Ar/70% He mixturethrough a pulsed slit valve 4 cm125 m, 10 Hz,0.5 ms. The best yields of Ar–H 2 O and H 2 O 2 complexesare achieved at a total stagnation pressure of 300–500 Torr,with the yield of dimer decreasing at higher pressures orat larger Ar fractions presumably because of preferentialformation of larger clusters. The v OH =2 overtone vibrationsof jet-cooled molecules are excited with a tunablenear-infrared pump laser =7100 cm −1 –7300 cm −1 ,upto20–30 mJ/pulse, 0.2 cm −1 resolution, 5 ns pulse width. Acounterpropagating ArF excimer photolysis laser pulse193 nm, 5 mJ/pulse, 7 ns pulse width follows after a variabletime delay 0–1000 ns with respect to the pump, dissociatinga fraction of vibrationally exited water moleculesFig. 1. Finally, a probe laser pulse 30 J/pulse, 0.1 cm −1 ,5ns excites the nascent OH on the off-diagonal A 2 ←X 2 v=1←0 band some 20 ns after the photolysis pulse,with the resulting OH fluorescence collected from the diagonalA 2 ←X 2 v=1←0 band at 310 nm. Both excitationand detection take place 2 cm downstream from the expansionslit. To discriminate between i vibrationally mediatedand ii direct 193 nm photolysis of H 2 O and its complexes,the near-infrared pump laser is operated at half the repetitionrate, with the data from alternate laser shots subtracted togenerate a background-free signal.The resulting OH fluorescence signal is found to be linearin the 193 nm photolysis laser power, indicating thatmultiphoton photodissociation processes in the parent moleculeare not relevant. The OH transitions are then probed inthe weak saturation limit and calibrated against fluorescenceexcitation spectra under fully thermalized conditions. On theother hand, the IR pump transitions can be saturated significantly,despite the decrease in absorption strength with successiveovertone excitation. Indeed, for the strongest overtonetransitions, it proves necessary to attenuate the pumplaser power by as much as two orders of magnitude to avoidpower-broadening of spectral lines beyond the specified laserresolution of 0.2 cm −1 . For optimal sensitivity, therefore,overview scans are taken under full near-IR pump laserpower, with scans of individual overtone bands taken underreduced power conditions.III. RESULTS AND DISCUSSIONA. Spectroscopic notationWe use mn ± local mode notation 44 for labeling OHstretchingvibrations of free H 2 O, Ar–H 2 O, and the monomerproton acceptor subunit in H 2 O 2 , where m and n arethe local mode-stretching quanta. 45 In this notation, one canapproximately correlate 1 + 3 and 2 1 normal mode statesof H 2 O with 02 − and 02 + local mode states, respectively.For the proton-donor unit of H 2 O 2 , the states are labeled byspecifying the number of local mode excitations in thebound-OH and free-OH bonds. 24 For example, 0 f 1 b designatesthe v OH =1 hydrogen-bonded OH stretch fundamentalvibration of H 2 O 2 .We use the notation of Ref. 10 for labeling rotationalstates of H 2 O 2 . Briefly, each rotational level of H 2 O 2 issplit into sextets by three internal motions: acceptor internalrotation, acceptor-donor interchange, and donor proton interchangeFig. 2. Internal rotation of the proton acceptor subunitis extremely facile, splitting each J,K state into widelyseparated “upper” and “lower” K manifolds e.g., 10 cm −1between K lower =0 and K upper =0, with acceptor-donor interchangeand donor switching resulting in more modest additionalsplittings of each K level into A 1,2 , E, and B 1,2 sublevelse.g., A 2 − and B 2 − are separated by 0.65 cm −1 in J=0,K upper =0. Furthermore, all K0 levels are split into dou-Downloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
194316-3 Overtone spectroscopy of H 2 O clusters J. Chem. Phys. 122, 194316 2005FIG. 2. Schematic diagram of H 2 O 2 energy levels not to scale and principalinertial axes for H 2 O 2 . Only K levels appreciably populated at jettemperatures are shown. Each K level is split into K lower and K upper componentsby proton acceptor internal rotation. Further splitting arises because ofdonor-acceptor interchange. For K0, there is an additional doubling of alllevels. The total symmetry including rotation in permutation-inversiongroup isomorphic to D 4h and nuclear weights for each level are given for atotally symmetric vibration of H 2 O 2 .blets by conventional asymmetry considerations. The K lower=1 and K upper =0 manifolds are close in energy because ofthe comparable tunneling and K a =1 rotational pathwaysaround the A axis. Finally, H 2 O 2 molecules under supersonicconditions cool down to the lowest levels within itsfive nuclear spin symmetry subgroups A 1 , E, B 1 , A 2 , and B 2 with a fairly large spacing 10 cm −1 between the A 1 , E, B 1and A 2 , B 2 manifolds. To the extent that all internal motion inH 2 O dimer is maximally cooled, one would quite simplyexpect comparable 7:9 populations in the K lower =0 vsK upper =0/K lower =1 manifolds.The energy levels of Ar–H 2 O are much more simplyrepresented in the framework of nearly freely rotating H 2 Oin the slightly anisotropic potential resulting from the Aratom. Specifically, ortho and para nuclear spin designationsare still good, and the energy levels of the complex can beconveniently labeled by quantum numbers of the free H 2 Orotational states J Ka K c with which they correlate. Ar–H 2 Olevels are additionally characterized by the projection of thetotal angular momentum on the intermolecular axis and bythe number of quanta in the intermolecular stretching mode s , as explained in Refs. 46 and 47. Figure 3 displays aschematic diagram of the lowest energy levels of Ar–H 2 Oadopted from far-infrared and near-infrared studies 11,42,47,48along with allowed transitions for 02 − band.B. Overview spectrumFigure 4 shows an overview spectrum recorded underconditions optimized for the maximal yield of Ar–H 2 O complexes.The spectrum is obtained by tuning the UV probelaser on the Q 11 8 line of the A 2 ←X 2 v=1←0 band thatprobes the 2 −3/2 N=8 rotational state of OH v=0 and thencontinuously scanning the near-infrared pump laser frequencyover the characteristic first OH-stretching overtoneregion. As will be elucidated below, the choice of a relativelyhigh-N state of OH for detection e.g., N=8 vs N=1 is usedto maximize action spectral intensities from complexes relativeto those from H 2 O monomer. In addition to a strongFIG. 3. Correlation between the Ar–H 2 O and H 2 O energy levels. The gridof the Ar–H 2 O levels is shifted with respect to that of H 2 O by the bindingenergy of the complex. For 02 − vibrational state, the ortho and para labelsare interchanged compared to 00 + on the account of the asymmetry of thevibration, with the 02 − ←00 + transitions following a-type selection rules:K a =even, K c =odd.dependence on OH probe state, band intensities in the spectrumare also affected by time delay between the pump andphotolysis laser pulses, because vibrationally excited complexescan undergo intermolecular predissociation beforeH 2 O molecules inside them are photolyzed Fig. 1. Indeed,this will serve as a basis for direct measurement of vibrationalpredissociation lifetimes for Ar–H 2 O and H 2 O 2 ,asdescribed later. The spectra in Fig. 4 are obtained with apump-photolysis delay chosen to be 500 ns; this effectivelyensures that all complexes predissociate prior to photolysisby the excimer laser pulse.As the first stage in the spectral assignment, only threerovibrational transitions of jet-cooled H 2 O appear in thisspectral range and with appreciable intensity; these correspondto J Ka K c=1 01 ←0 00 para and 0 00 ←1 01 and 2 02←1 01 ortho transitions in the 02 − vibrational overtoneband. A few H 2 O monomer transitions into 02 + state alsooccur in this spectral range but are considerably weaker andindeed undetectable at the current S/N in Fig. 4. The remainingbands in the spectrum cannot be attributed to free H 2 Olines and, therefore, must belong to complexes containingH 2 O. Note that these bands are of comparable intensity tovibrationally mediated water monomer lines. This is not areflection of water clustering efficiency but rather that detectionon high OHN states provides an enormous discriminationagainst water monomer, obviously present in muchhigher concentrations.Many bands in the action spectrum are strongly correlatedwith fractional Ar content in the expansion mixture,suggesting complexes between H 2 O and Ar Table I and Fig.Downloaded 26 May 2005 to 128.200.198.26. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
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