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Order and Symmetries 

of Sexithiophene 

within Thin Films Studied by 

Angle-Resolved Photoemission 

Diplomarbeit 

Freie Universität Berlin 

Fachbereich Physik 

vorgelegt von 

Cynthia Heiner 

aus New York, USA 

Berlin 2004

Gutachter dieser Arbeit: 

Prof. Dr. I. V. Hertel 

Die Arbeit wurde durchgeführt am 

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie Berlin

Abstract 

The present work is concerned with the electronic structure and molecular orientation of 

thin films of sexithiophene (6T), a technologically promising organic conjugated molecule. 

Using a Au(110) single crystal substrate to tailor a well-controlled initial self-assembly of the 

6T molecules (long axis) along the troughs, well-ordered 6T thin films were grown and maintained 

up to ca. 2000 Å. Angle-resolved photoemission from these films, performed at the 

MBI undulator beamline at BESSY, show a strong intensity dependency on the experimental 

geometry. For the present experiments it was, however, necessary to use simultaneous laser 

irradiation to compensate for film charging, in order to obtain sharp (and resolved) spectral 

features. Additionally, quantum chemical calculations were performed on various simulated 

6T isomers (conformers) to compare molecular potential and orbital binding energies. Then 

by interpreting the data through symmetry selection rules, the molecules’s relative orientation 

was inferred in conjunction with the various molecular orbital character possibilities. 

The results firmly suggest the need for alternative molecular symmetries at the surface of the 

film as compared to the bulk crystalline structure, namely C 2v and C s vs C 2h , respectively.

Contents 

1 Introduction 3 

2 General Background 6 

2.1 Fundamental Aspects of Photoemission . . . . . . . . . . . . . . . . . . . . . 6 

2.2 Synchrotron Radiation from Insertion Devices . . . . . . . . . . . . . . . . . 11 

2.3 Thiophene Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

2.4 Symmetry Point Groups Relevant for 6T Thin Films . . . . . . . . . . . . . 17 

2.5 Hartree-Fock Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

3 Experimental 27 

3.1 MBI-BESSY Beamline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

3.2 UHV Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

3.3 Materials and Film Preparation . . . . . . . . . . . . . . . . . . . . . . . . . 33 

4 Results and Discussion 35 

4.1 6T Film Characterization: Growth pattern, Thickness, and Morphology . . . 35 

4.1.1 Low Energy Electron Spectroscopy (LEED) . . . . . . . . . . . . . . 35 

4.1.2 Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . . 41 

4.1.3 X-Ray Photoelectron Spectroscopy (XPS) . . . . . . . . . . . . . . . 44 

4.2 Photoemission Spectra and the Need for Laser Excitation . . . . . . . . . . . 47 

4.2.1 6T Spectral Evolution and Assignment Through Coverage . . . . . . 47 

4.2.2 Film Charge Compensation in the Presence of Laser Irradiation . . . 51 

4.3 Angle-Resolved Photoemission from 6T/Au(110): Molecular Orientation . . 56 

4.3.1 Measured Photoemission Anisotropy . . . . . . . . . . . . . . . . . . 56

CONTENTS 2 

4.3.2 Calculated Molecular Orbitals: Binding Energies, Characters, and 

Symmetries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

4.3.3 Photoemission Data and Computation Comparison . . . . . . . . . . 70 

5 Conclusions 79 

6 Appendix 81 

6.1 Laser Systems Used in Combination with Synchrotron Light . . . . . . . . . 81 

List of Figures 83 

List of Tables 86 

Bibliography 87

Chapter 1 

Introduction 

In recent years investigations on systems of π-conjugated organic materials, both polymers 

and oligomers, have attracted quite a bit of attention owing to their prospects for new technological 

applications and devices, and their interesting physical properties. Among these are 

particularly polythiophene and its n-thiophene derivatives [1], which have a promising value 

for both photonics, including e.g. organic field-effect transistors (OFET) [2], lightweight batteries, 

electrochromic devices [3], and optoelectronics, such as organic light-emitting diodes 

(OLED) [4], fast optical switches [5], and solar cells [6]. Of course, to realize the full potential 

applications (and device performance) in these electronic or optical devices, it is necessary 

to have a good understanding of both the electronic structure and physical properties of 

the system. The latter not only depends on the physical properties of the individual organic 

molecules but also on their relative orientation, which in turn affects their mutual 

interaction and thus fundamental film properties, e.g. charge transfer processes and various 

relaxation mechanisms. Thus the creation of highly-ordered organic thin films becomes a 

very compelling topic to study. 

Sexithiophene, which is the subject of the present study, was originally investigated as 

a model system to better describe its parent 1 , polythiophene [7]. Sexithiophene, 6T, is a 

planar, aromatic organic molecule consisting of six conjugated thiophene rings - each ring 

1 Well defined (and then highly-ordered) films of polythiophenes are very difficult to achieve, in part 

because within the polymerization process, the polythiophenes are likely to develop chemical and/or morphological 

defects, e.g. intermolecular entanglements and chain twists, causing break in the electronic conjugation. 

This fractioning leads to an assortment of conjugation lengths within the already non-crystalline 

(amorphous) polymer structure [11], [12], [13], [14].

4 

possesses four carbon atoms and one sulfur atom. After some initial studies on 6T, however, 

it became quickly clear that the molecule itself was interesting in that the material (films) 

could be well controlled and prospectively tailored (chemical structure changes such as side 

or end substitution [7], [8]) for technological applications. In fact, 6T has since been already 

used for fabricating devices [9]. Specifically, good control over 6T thin film morphology can be 

practically achieved through convenient processing techniques, most commonly using vacuum 

sublimation under ultra-high vacuum (UHV) conditions [7]. Clearly the interfacial (organic 

film - substrate) properties are also crucial; using UHV is an advantageous technique here 

since it assures that no contaminants could interrupt the relationship between the deposited 

molecules and the substrate [10]. This provides for suitable control of the interface; for our 

purposes this is of particular importance since the substrate serves as a seed to induce a high 

structural order to be built up from submonolayer coverage to thin films. This opens the 

way to a variety of surface science techniques, some of which are applied within this work 

to characterize the 6T film growth. 

In the present work the electronic structure of highly oriented sexithiophene is studied 

using angle-resolved photoemission. The main goal of this study is to infer the structural 

and electronic information through symmetries and binding energies, respectively, of 6T 

molecules within well ordered thin films. Films were grown on a (1x2)-Au(110) singlecrystal 

surface providing a suitable ordered template for highly-orientated molecular (film) 

growth. The molecules prefer to nucleate with their long molecular axes along the [110] 

direction of the single crystal substrate. Continual deposition of 6T leads to island formation; 

however, these islands still posses the same aforementioned preferred orientation within the 

macroscopic film. This structure leads to a highly anisotropic emission of photoelectrons. 

Presented here are valence band photoemission spectra of thin, well-ordered sexithiophene 

films grown on Au(110) obtained, for 50 eV photon energy, at the MBI-BESSY Undulator 

beamline (SGM-U125). We were able to compensate for the large amount of synchrotron 

induced film charging, typical for low-conducting organic films, by employing simultaneous 

laser irradiation. This was necessary, since only in the presence of laser pulses will the 

resulting spectra posses sharp features. Furthermore, we show through angle-resolved photoemission 

a strong variation of photoemission intensity as a function of the experimental

5 

geometry, i.e. the direction of the polarization vector of the incoming excitation-light, and 

the angle of photoelectron detection. 

The main goal of this latter series of experiments is to indicate the 6T molecular orientation. 

This is inferred by applying group theory and symmetry-derived selection rules directly 

to the angle-resolved PE data, to examine the permission of detection of photoemitted electrons 

from 6T films. By performing parallel quantum-chemical calculations on simulated 

(optimized) molecular isomers (conformers), we determined the correlating (molecular orbital) 

binding energies, and their respective symmetries, as depending on various 6T molecular 

symmetries. The molecules’ resulting relative orientation, after taking into account the 

differences between experimental and theoretical frameworks, is discussed in relation to these 

various conformers. Notice that photoemission, for photon energies of 50 eV, is a reasonably 

surface sensitive technique. Our results suggest a different molecular orientation found at 

the surface as compared with previously reported 6T’s bulk-crystalline structure.

Chapter 2 

General Background 

The main experimental technique applied within the present work, photoemission (PE), will 

be briefly revisited (Sec. 2.1). Other techniques used, e.g. Low Energy Electron Diffraction 

(LEED) and Atomic Force Microscopy (AFM), will not be described here explicitly. Also 

included is a general description of synchrotron radiation (SR) obtained from an undulator 

(Sec. 2.2), since the majority of PE experiments were obtained using SR light. Presented in 

Sec. 2.3 is an overview of sexithiophene’s chemical and physical (electronic) properties, with 

an emphasis on the condensed phase. Additionally, a short summary of important aspects 

of aromatic organic molecules, specifically n-thiophenes, and their interactions (bonding) 

with various metal surfaces for both thin film, sub- and monolayer systems. A particular 

focus will be on the molecular orientation within the latter systems. After this general 

introduction we will focus on the symmetry details of three reported 6T conformers. This 

becomes relevant for interpreting the present PE results by applying group theory, the basics 

of which are reviewed in Sec 2.4. Lastly, the theoretical model employed here for binding 

energy calculations - the Hartree-Fock model including the basis set 6-31G(d,p) used here - 

will be discussed (Sec. 2.5). 

2.1 Fundamental Aspects of Photoemission 

One of matters’ basic characteristics is that of its electronic structure. This is most commonly 

probed by invoking the photoelectric effect [15] in the experimental technique of 

photoelectron spectroscopy (PES). Essentially, using photons with a known energy (hν) to 

excite a system, emitted electrons can be detected and analyzed according to their respective

2.1 Fundamental Aspects of Photoemission 7 

kinetic energy to yield direct individual electronic state information. To guarantee a known 

(precise) incoming energy, highly monochromized light is wanted and can be fully exploited 

by synchrotron radiation providing photons ranging from the near-ultraviolet (UV) to far 

x-ray regimes. The former technique is then called UV Photoelectron Spectroscopy (UPS) 

and the latter either X-ray Photoemission Spectroscopy (XPS) or Electron Spectroscopy for 

Chemical Analysis (ESCA). This technique can be performed on atoms and molecules in 

gas, solid, and liquid phases; however, it should be noted that special technical requirements 

are necessary for PES from highly volatile liquids 1 , which have been achieved and detailed 

elsewhere [16]. 

There are two main interpretations of the photoemission process in solids. The simple 

so-called three-step model breaks the process up into three distinct stages [17], [18], [19]. 

First the local absorption of a photon excites an electron. Next the excited electron must 

travel through the sample medium to the surface, until it reaches the third step; its escape 

into the vacuum where it is detected. Inherently these three steps would be dependent upon 

each other and also on the other N electrons within the system; therefore this model is a 

somewhat naive and artificial picture of the photoemission process. Thus, a one-step model 

is a better theoretical approach, albeit more rigorous. 

The absorption of a photon of energy (hν) by an initial system of N electrons, which is 

described by ψ i (N) and E i (N), will optically excite an electron (possibly photoionization). 

After photoionization, the system can be described by its remaining N-1 electrons, in a final 

state of ψ f (N-1, k) and E f (N-1, k) and the missing electron; k depicts the initial energy 

level from which the electron was removed. Here the escaped electron will carry information 

of its binding energy, via its kinetic energy E kin [17]. Energy conservation then renders this 

relation as 

E i (N) + hν = E f (N − 1, k) + E kin (2.1) 

This can be re-arranged to show the binding energy with respect to the vacuum: 

E B (k) = E f (N − 1, k) − E i (N) (2.2) 

To reiterate, the final state wave function is that of an atom with a (positive) hole in level 

1 Special parameters must be considered to overcome the difficulties of operating a liquid system in ultra 

high vacuum conditions (required for photoemission spectroscopy.)


nucleus 

inner levels 

valence band 

hν 

hν 

hν 

binding energy 

E B = hν - E kin 

kinetic energy 

of photoelectrons, E kin 

E = 0 

Fig. 2.1: Illustration of photoemission spectroscopy [20]. Here the initial shell (density of states) 

where the electron arises from is shown as dashed lines. These electrons are excited by the incoming 

energy hν, which is the difference between the electrons’ binding and kinetic energies. 

k. Figure 2.1 visually relates the above equations to the observations made in photoemission 

spectroscopy [20]. 

First we will assume that after the photoionization process the remaining N-1 electrons in 

the excited ion, and the N electrons in its neighboring atoms do not re-arrange to compensate 

for the electrical imbalance left behind by the escaped electron. This would be more accurate 

however, in examining an infinite isolated atomic or molecular system. In such a case one 

would say that the identical wave function describes both the initial, ψ i (N), and final, ψ f (N- 

1, k), states, which then describes the orbitals by the frozen orbital approximation. Within 

the Hartree-Fock approximation this leads to Koopmans’ theory - that the binding energy 

is the same as the negative orbital energy of the emitted electron (ε k ) [21]: 

E B (k) = −(ε k ) (2.3) 

This assumption - that the ejection of a photoelectron is much faster than the time needed 

for the systems’ valence electrons to re-arrange their charge distribution - encompasses the 

appropriately named sudden approximation [18], [19]. 

However, in the other scenario of the more realistic non-isolated system there is a (fast) 

response to the new electronic environment by the remaining N-1 charges; they try to readjust 

in such a way as to minimize their energy (relaxation). This relaxation process is then a final 

state effect resulting in an extra positive energy value (E R ), which must be incorporated into


equation 2.3 to correctly evaluate the initial state binding energy [17]: 

E B (k) = −(ε k ) − E R (k) (2.4) 

Now there is a clear difference between the initial and final wave functions, which has been 

appropriately taken into account. This assumption - in which the photoelectron leaves the 

system slowly enough that the electrons from or near the excited atom change their energy 

by adjusting to the effective atomic potential in a self-consistent way (embodied by E R ) - is 

referred to as the adiabatic approximation. 

It is important to note at this time that the interactions between the remaining electrons 

after ionization are very fast, and a universal agreement for an ”instantaneous” time-scale 

is difficult to define. The transition probability, again, depends on the wave functions; thus 

it is crucial to know if one has the same or dissimilar initial and final states, or essentially 

when to apply which aforementioned approximation(s). There are no strict criteria for the 

favored applicability of one approximation over the other in certain defined energy ranges. 

The time for the PE process depends on the velocity of the escaping electron and thus 

the sudden limit corresponds to high excitation energies. The sudden approximation only 

becomes exact in the limit when the photoelectron’s kinetic energy is infinite, as was reviewed 

here for an isolated system. Furthermore, the adiabatic approximation, which considers a 

photoelectron leaving the system slowly, corresponds to low excitation energies. 

Both theoretical and experimental investigations devoted to the estimation of the kinetic 

energy limit between the two approximations are not yet entirely conclusive. Results from 

Hedin and Johansson [22] are recommended for the more in-depth reader. It provides details 

concerning the transition between the approximations and examples of order of magnitudes 

for sp-metals. Other studies and calculations by Gelius [23], applying an optimized Hartree- 

Fock-Slater method, breaks the PE process into inter-, intra-, and extra-atomic relaxations. 

As previously described, the photoemission process is essentially the excitation of an 

electron from an initial state to a final state. This excitation is triggered by an electromagnetic 

field (UV/X-ray) whose interaction with the initial and final wave functions is the 

intensity of the measured PE signal (photocurrent). Describing the system’s ground state 

by the Hamiltonian H 0 and the ionizing photon beam as H’, the transition probability per


unit time between the two eigenstates ψ i and ψ f is given by Fermi’s Golden Rule [18] 

R = 2π¯h |〈Ψ f|H ′ |Ψ i 〉| 2 δ(E f − E i − ¯hω). (2.5) 

The perturbation (using the first-order perturbation theory [24]) is then expressed by [17] 

H ′ = 

e · (Ap + pA), (2.6) 

2mc 

where P is given by the momentum operator -ih∇ and A being a plane wave, A(r,t)=A 0 exp(- 

iωt+iqr), describing the vector potential of the incident light. This operator equation can 

be reduced, by involving the dipole approximation [20], [24], to form a final perturbation of 

H ′ = −→ E · −→ µ (2.7) 

Within the approximation the vector potential A collapses an electric field −→ E , and −→ µ is 

the dipole moment 2 . 

Also, it is important to note that UPS is a reasonably surface sensitive technique [25]. 

In handling the complex, interconnected system of atoms (i.e. a non-isolated case such as a 

solid), the strong interactions of the electrons with matter must be taken into account. An 

electron travelling through a solid has a specific probability of escaping into vacuum without 

any inelastic scattering events causing the electron to suffer energy losses. Only electrons 

that escape from the solid collision-free, carry direct information of the electronic structure. 

This certain information depth is known as the mean free path or the electron escape depth, 

and is a function of the electron kinetic energy. 

This relation is known as the universal 

mean free path curve 3 , displayed in Fig. 2.2. The minimum of this curve, at ca. 20-100 eV, 

refers to the energy range of maximum surface sensitivity (lowest escape depth). The large 

number of electrons that originate from distances larger than the electron escape depth (on 

the order of a few monolayers) will undergo inelastic scattering events, which results in the 

secondary electron background signal in PES. 

2 The measured effect here can be also calculated by the application of group theory (see background Sec. 

2.2). These two methods have one important factor in common: Altogether the intensity of the photoemission 

signal depends on the transition probability given by the initial and final wave functions, determined by the 

wave function’s respective character. 

3 The quasi-universal behavior of electron mean free path, regardless of material, comes from the major 

interaction mechanism between electrons and a solid. That is to say the excitation of plasmons; this is a 

fundamental interaction of the electron density of solids, which is very similar in most cases.

2.2 Synchrotron Radiation from Insertion Devices 11 

1000 

mean free path [monolayers] 

100 

10 

1 

1 10 100 1000 

kinetic energy [eV] 

Fig. 2.2: The universal mean free path of electrons [17], referring to the the electron escape depth 

in solids, as a function of kinetic energy. 

2.2 Synchrotron Radiation from Insertion Devices 

A brief look at the advantages of synchrotron radiation (SR), as the photon source of most 

of the present PE measurements, and how it is of particular use related to this work will 

be reviewed. This is admittedly not only non-exhaustive but presented with a very narrow 

scope pertaining only to the present experiments performed with undulator radiation. 

Synchrotron radiation is emitted when charged relativistic particles, electrons for instance, 

are subjected to centripetal acceleration. In a synchrotron, accelerated electrons are 

forced into a circular orbit by magnetic fields 4 . These relativistic electrons (v/c = β ≈ 1) 

radiate light tangentially to their orbit (in the forward direction) in a narrow cone [18]. The 

higher the kinetic energy of the electrons the narrower the emission cone becomes, while 

likewise extending the spectrum of the emitted radiation to higher energies. The spectral 

intensity of synchrotron radiation is characterized by its brilliance [24] 

B = 

spectral flux into 0.1% bandwidth 

source area (mm 2 ) · solid angle (mrad 2 ) · 100(mA) . (2.8) 

A further important feature of synchrotron radiation is its linear polarization, which is the 

key property for the present experiments. 3 rd generation synchrotron facilities use insertion 

devices (undulators or wigglers) - magnetic arrays (N periods with period length λ 0 ) within 

the straight sections of the storage ring - to achieve much higher brilliance as compared 

4 The word ”synchrotron” evolved from the idea of synchronization of magnetic fields and photon energy.

2.2 Synchrotron Radiation from Insertion Devices 12 

Fig. 2.3: Schematic of the magnetic structure of a wiggler/undulator [26]. The distance between 

the magnets (gap) is labelled as h. Here the electrons’ propagation is along the z-direction (planar) 

and its oscillation within the x-z plane. Radiation is emitted (in the forward direction) at each 

curve; the total of these individual intensities increases with the number of poles in the electron 

path. 

to simple bending magnets. The magnetic structure of a wiggler/undulator 5 is shown in 

Figure 2.3. Specifically, for an undulator defined by a characteristic K (≈ 0.934B 0 λ 0 ≈ 1) 

parameter, which is not further detailed here (see [24]), light emitted from each individual 

periodic magnetic structure adds up coherently 6 . This causes the spectral intensity to be 

confined around a certain wavelength (quasi-monochromatic) at highly increased brightness 

(see [24]). The wavelength of the radiation can be adjusted by the magnetic field strength, B 0 , 

of the device, which is performed by changing the distance between the magnet poles, referred 

to as changing the undulator gap. For a planar undulator the light is linearly polarized, 

in the direction of the oscillations of the electrons’ trajectories; circularly or elliptically 

polarized light can be generated by specially arranged magnetic structures. Finally, modern 

synchrotron sources typically deliver pulse widths of only 30-50 ps 7 , which can be directly 

used for time-resolved experiments. A special application is the combination of short laser 

and synchrotron pulses in a pump-probe experiment as addressed in Sec. 6. 

5 Undulators and wigglers are in principle the same; they differ in the magnetic field values - e.g. wigglers 

produce higher magnetic fields but have fewer magnetic poles than undulators. 

6 Constructive interference results from the overlap of the light cones of the individual wiggles and electromagnetic 

waves emitted from the same electron at different positions as it passes through the magnetic 

structure. 

7 Even smaller pulse widths, ∼ 1 ps, are also available in the low-α mode [27]

2.3 Thiophene Molecules 13 

2.3 Thiophene Molecules 

This section is intended to give an introduction to the molecular architecture of oligothiophenes 

(OT). Additionally, an overview on the molecules’ interactions with particular 

substrates, specifically Au (since this substrate was used for the present experiments), and 

the implications of these interactions with respect to the molecules’ orientation in the monolayer 

regime and in multilayer films is presented. Optical and electronic properties are only 

touched upon here, since the electron energies of 6T are investigated in further detail in Sec. 

4.3.2. 

Thiophene is a five-membered sulfur-containing aromatic compound, as sketched in Figure 

2.4. The addition of adjacent monomer units, each bonded via the α-carbons - i.e. the 

carbons neighboring the sulfur atoms [28] - results in oligomers (small chain length polymers) 

of thiophene (nT). In our case, we have a chain of six thiophene monomers. The two carbon 

atoms opposite of the sulfur, known as β-carbons [29], [30], are not affected by the chain 

length (since they do not directly interact, e.g. in a bridge position, with the neighboring 

rings). Thiophene rings can be conjugated as cisoid (all sulfur atoms on the same side, then 

all-cis) or transoid (all alternating, then all-trans); these gas-phase conformations differ by 

180 ◦ dihedral (torsion) angle, and are very similar in energy (see Sec. 4.3.2). The situation 

may be different for adsorbed molecules, where this small energy barrier [31], [32] may be 

overcome by involving the carbon and sulfur atoms in surface bonding [6]. 

It is exactly 

this surface-bonding mechanism, which varies depending on substrate, that determines the 

orientation of the seeding monolayers, and is thus crucial for the formation of long-range 

ordered multilayers of OTs (and other large organic molecules) on crystalline surfaces. 

Metal substrates are then appreciable templates as they may provide considerable covalent 

interactions 8 with the organic molecule, and particularly the lateral ordering, at least 

in the monolayer regime, results from the interplay of both the substrate-adsorbate and the 

adsorbate-adsorbate interactions [33]. (These interactions may also result in different molecular 

isomers being found in the film than those expected (energetically favored) in the gas 

8 The bonding mechanism between nT’s and metals (noble and transition) is generally accepted to strongly 

involve the substrate d bands with a charge transfer from the molecular π-states to these d-bands, with an 

(almost) simultaneous back donation to the π ∗ states of the molecule [34].


S 

H 

C α 

C α 

H 

C β 

H H 

6 

Fig. 2.4: Sketch of a thiophene monomer with the atomic nomenclature for the carbon atoms. 

Sexithiophene consists of six conjugated rings. 

C β 

phase; this is something that will be thoroughly addressed in the present work (Sec. 4.3.2-3).) 

Finally, in most cases, this balance of interactions leads to commensurate superstructures 

for the first organic monolayers. Additionally, nT’s were found to adsorb nondissociatively 

on many noble metal surfaces; in fact the bonded molecules still exhibit sufficient lateral 

mobility for two-dimensional ordering. Mobility only, however, doesn’t necessarily warrant 

the formation of an ordered adsorbate monolayer. The substrate must additionally act as 

a template, e.g. through a particular corrugation structure that forces the assembly of the 

overlayer. Furthermore, any degree of order also sensitively depends on the experimental 

parameters (e.g. deposition rate [35]). 

Specifically on the Au and Ag single-crystal surfaces, the molecules tend to adsorb in a 

coplanar geometry (ring planes parallel to the surface); this is attributed to a preferential 

bonding to the metal via the conjugated π-system 9 [33], [36]. 

Parallel geometry is often 

taken as an indication of bonding via the ring π-electrons 10 , which may be identified in 

photoemission by peak shifts of the π-orbital features, or differential shifts of the σ-features, 

toward higher binding energies (see also Sec. 4.2.1). This so-called π-stabilization is a general 

feature of π bonding of aromatic molecules to metallic surfaces. In fact, although nT’s have 

been studied on various substrates (not only metallic), the question of chemical interactions, 

and likewise of other bonding mechanisms, is still highly debated 11 . 

9 The sulfur electrons play no important role in bonding [36]. 

10 A parallel geometry does not necessarily define π-stabilization. Both benzene and bithiophene on Al(111) 

adopt a flat-lying geomerty, however, they are bonded via a purely electrostatic mechanism (no bonding of 

either the π-electrons or the S lone pair) [28], [34]. 

11 For example, 4T on Ag(111) has been reported to have a weak coupling [36] but also a covalent 

(chemisorptive) [33] bonding character. Also, on Ni(110) and Cu(110) strong π-bonding is observed, which 

can then be de-activated, in both cases, by a sulfur-modification (passivation) of the surface [30]. However, a


a 

a 

b 

c 

b 

c 

Fig. 2.5: Stereographic view of the 6T unit cell, as inferred from x-ray diffraction measurements. 

This herringbone packing structure is very common among planar organic molecules [41]. 

Even for the present system, 6T/Au(110), theory and experiment are in disagreement 

as to the bonding mechanisms at the Au-nT interface 12 [37], [38], varying between strong 

π-bonding to weak electrostatic interactions (similar strength to Van-der-Waals) - we will 

detail this in Sec. 4.2.1. Yet, the 6T overlayer on this substrate has been shown to act as a 

template for the planar and orientationally ordered growth of at least five monolayers [39]. 

However, reasonable order can still be maintained for films as thick as ca. 2000 Å, which is 

one of the focuses of the present work. 

In the solid state (and thin films), nT molecules (n > 3) are almost planar (small tilt 

angle), with the point group symmetry C 2h for even numbered oligomers or C 2v for odd 

numbered oligomers [29]. 6T molecules usually crystallize in a herringbone (HB) structure, 

common to planar molecules, in which the molecules face each other forming a quasi bidimensional 

H frame [40]. The unit cell for crystalline 6T is presented in Figure 2.5 [41]. 

Another helpful illustration of the 6T HB structure is presented in Figure 2.6 as shown 

through space-filling molecular orbitals [42]. Orientational disorders of the molecules in the 

HB lattice were shown to strongly affect the crystals’ optical and electronic properties [40] 

(quenching optical signal and breaking the π-electron system). Hence it is desirable to have 

further S-modification (from a c(2x2)S and p(4x1)S surface) posses a still more reactive surface bonding [6]. 

12 It is worthy to note that this nT-substrate interaction, even when being a weak interaction, is still 

proposed to be responsible for the likely reconstruction of the substrate atoms, specifically for Au(110) and 

Al(111) surfaces [28], [34], [39].


20.00 Ångstroms 

Fig. 2.6: Space filling sketch of the herringbone pattern of the crystallized 6T lattice packing [42]. 

control of the arrangement of molecules (not only at the interface), which would affect their 

mutual distance and thus their interactions, within a thin film (solid) sample. Again, this 

intimately depends on the experimental parameters. 

Lastly, a brief look at the optical and electrical properties of 6T. The optical absorption 

spectrum of a thin film (ca. 50 nm) 6T/quartz 13 is presented in Figure 2.7. Since laser irradiation 

is used within this work to compensate for film charging (Sec. 4.2), the Ti:sapphire 

wavelength (second harmonic) is marked in the figure to guide the reader’s eye. Note that 

the absorption at λ = 400 nm lies in the middle of an absorption plateau, stretching between 

ca. 370 - 460 nm. It is worth mentioning that this absorption spectrum corresponds well 

with those found in literature [1], [43], [44]. Moreover, the electronic structure of 6T, particularly 

the π-orbitals (these are the orbitals important in surface chemical bonding), consists 

of a pair of localized and delocalized molecular orbitals per thiophene ring 14 (bonding and 

anti-bonding, see Sec. 2.5) [29], [31]. Here localized and delocalized refer to the probability 

of locating the electron wavefunction anchored on one atom (localized) or free to roam 

among several neighboring atoms (delocalized), specifically the carbon atoms. For the sp 

hybridized carbon atoms within 6T, there exists three σ-bonds and one remaining p-orbital, 

which overlaps with the adjoining unsaturated p-atomic orbital to form the continuous π- 

network (hence the term conjugated polymer) [45]. Figure 2.8 visually depicts the delocalized 

and localized orbitals, in 2-D and 3-D on the left and right sides respectively, according to 

13 Film was grown under identical deposition parameters as those grown on Au(110), which were used in 

the present experiments. 

14 The valence orbitals develop gradually from molecular orbitals into broad bands in the large length 

limit [31].

2.4 Symmetry Point Groups Relevant for 6T Thin Films 17 

0,7 

0,6 

223 

ca. 50nm 6T/Quartz 

Absorbance [arb. units] 

0,5 

0,4 

0,3 

0,2 

0,1 

277 

366 

de-charging 

laser 

448 

474 

517 

0,0 

200 300 400 500 600 

Wavelength [nm] 

Fig. 2.7: Absorption spectrum of thin film of 6T on Quartz. The first excitation band includes 

four peaks; the wavelength of the de-charging laser (marked) lies in the middle of this band. 

the location of the carbon double and single bonds 15 . Notice that when the electronic structure 

consists of carbon double bonds connecting the C α -C β (top-left) the π-electron system 

likewise extends between the rings (top-right). This is known as aromatic character; clearly 

here no contribution stems from the sulfur atom. When the sulfur p z orbital does participate 

in the molecular orbital (bottom-right), it remains separate from the delocalized carbon 

bonds, the C β -C β double bond (bottom-left) - i.e. localizing the electrons to one thiophene 

unit. This is denoted as quinoid character, always with significant electron density on the 

sulfur and (almost) no density on the C α ’s. 

2.4 Symmetry Point Groups Relevant for 6T Thin Films 

Within the scope of this work group theory will be applied, in the form of selection rules, to 

examine the permission of detection of photoemitted electrons from 6T films. The selection 

rules are based on the innate symmetries of the molecule. 

It is important to note that 

symmetries may be different for molecules adsorbed at a single-crystal substrate vs the 

multilayer/vacuum interface. A small review of the implications driven by symmetries will be 

presented here; for a more in-depth look into this topic the reader is referred to [46], [47], [48]. 

15 The backbone of this structure (alternating double and single carbon bonds) resembles that of polyacetylene, 

a model organic molecule, however, with the sulfur atom breaking the symmetry [31].


S 

S 

aromatic 

(delocalized) 

quinoid 

(localized) 

S 

S 

Fig. 2.8: The two types of molecular orbital characters for thiophenes: aromatic vs quinoid. The 

particular pattern of the carbon double and single bonds dictate the localization of the orbitals’ 

π-electrons. 

Molecules themselves have innate symmetry elements, such as mirror planes or rotational 

axes, which likewise applies to the electronic wave functions (orbitals). An operation that 

moves the entire molecule into a new position that is identical to its original one is known 

as a symmetry operator. 

There are four symmetry operators pertinent to this work: the rotation of a molecule 

about any fixed axis, C, with a rotation angle, 2π/n, leads to the expression C n . A reflection 

through a plane (mirror symmetry) is denoted by σ; in our case σ may carry a subscript of 

either v or h to indicate a vertical or horizontal plane, respectively. An inversion, i, takes 

each atom of a molecule and passes it through the molecule’s center to affix it on to the 

opposite side of the molecule. Lastly the identity - wherein essentially nothing happens to 

the molecule - would, of course, produce an identical image; this is marked by either E or I. 

All molecules can be filed according to their symmetry elements into point groups that fulfill 

the highest number of symmetry operations (wherein the operators are already manipulated 

into mathematical groups 16 ). 

Here is a quick familiarization for the point groups addressed throughout this work. 

Although C 1 is officially a point group it actually defines a molecule with no symmetry. 

16 Any mathematical group possesses four basic properties: the existence of an identity and an inverse 

(reciprocal), the associative property between any two operators, and that the product of any two operators 

results in an operator contained within the group.


C 2 

axis 

Highest symmetries 

C 2 

axis 

C 2v 

C 2h 

σ v 

Reduced symmetries 

C 2 

axis 

σ h 

C s 

C 2 

σ h 

No symmetry 

C 1 

Fig. 2.9: Oligothiophenes isomers belonging to the various symmetry point groups pertinent to 

this work. The higher symmetry groups are shown (top) as well as each of these (C 2h and C 2v ) 

symmetry reductions, possessing a broken horizontal or vertical mirror plane, respectively. 

Often such a situation arises for a molecule that initially possesses a higher symmetry but is 

deformed, for example a bent or broken molecule, and must reduce its symmetry to C 1 . The 

next two symmetries, C s and C n , are also common symmetry reductions. They each contain 

only one symmetry operator that leaves the molecule invariant, namely a plane of reflection 

for C s and an n-fold axis of rotation for C n . The most complex symmetries investigated 

here, albeit still relatively low symmetries, are C nv and C nh , which contain both mirror and 

rotational symmetries. C nv can only have vertical mirror planes that are situated collinear to 

the rotation axis whereas C nh maintains only horizontal mirror planes that are perpendicular 

to the C n axis. For our purposes the rotational angle around the C n axis will always be 180 ◦ 

degree so it will be further referred to simply as the C 2 axis. 

All of these symmetries, possessed by oligothiophene isomers, are drawn in Figure 2.9 

to aid the reader in visualizing the aforementioned symmetry elements and operators. In 

Figure 2.9 the two highest symmetries seen in this work are C 2v and C 2h , shown in the upperleft 

and upper-right corners of the sketch, respectively. The C 2v bithiophene will return to an 

identical position when rotated into the page of the paper, or around the C 2 axis (red), which 

is (and must be) collinear to the one existing vertical reflection plane (σ v ) extending along the 

y-z axis. Clearly thiophene rings have only one sulfur atom that has no mirror image across 

the molecule’s x-z axis, rendering this plane as a low symmetry axis. The implications of this


will be described in the results and discussions (Sec. 4.3.3). Furthermore the constructed C 2h 

bithiophene also has a C 2 axis where the rotation of the molecule occurs in the plane of the 

page, or the C 2 axis is directed out of the page, and perpendicular to it lies the mirror plane. 

This defines a symmetry above and below the molecule itself. Instinctively one notes then 

that any slight defect in the linear alignment of the molecule would cause the symmetry 

plane to break. If all the rings were twisted out of plane, but twisted congruently by an 

exact angle, then there would still be a reduction in symmetry for the entire molecule, but 

it would only be to a C 2 symmetry (middle, right). It is worth noting that a C s symmetry 

is another possibility (middle,left); this occurs when the molecule remains planar but (1) 

one of the rings is ”flipped” (a 180 ◦ degree rotation around the dihedral angle) breaking 

the vertical mirror symmetry, or (2) the plane where the molecule exists is tilted at some 

angle with respect to a normal coordinate system. This latter symmetry may account for 

the herringbone molecular structure observed in bulk 6T (Sec. 2.3). Moreover for 6T, the 

C s symmetry can exist for twenty different planar conformers, e.g. with all the rings except 

one with identical positioned sulfur atoms. A defect that occurs when the entire molecule 

is broken or rearranged into a completely asymmetrical conformation (and probably not 

planar) would subject the molecule to a further symmetry reduction to a C 1 point group 

(bottom). 

Furthermore, all the symmetry operators (within a point group) are conventionally represented 

in a matrix form, and within this are the characters reduced into their irreducible 

forms. In our cases this refers to a block (square) matrix of sets of (1x1) matrices. Applying 

the symmetry operators of any point group, which of course leaves the molecule invariant, 

to all molecular orbitals, that is to say the Eigen functions of the symmetry operators, determines 

whether the orbital is symmetric or anti-symmetric (with respect to the symmetry 

operations). Mathematically speaking this would define 

Sψ = ±1ψ, (2.9) 

where S is the symmetry operator acting upon the Eigen function ψ, and +1 and -1 

represent symmetric and anti-symmetric states, respectively. The (1x1) matrix combinations 

from 2.9 can be summarized into character tables. The character tables for the symmetries 

reviewed here are shown in Tables 2.1- 2.5.


C 1 I Polarization 

A 1 T z , T x , T y 

Tab. 2.1: The universal C 1 symmetry character table. In this lowest symmetry emission is always 

permitted. 

C 2 I C 2 Polarization 

A 1 1 T z 

B 1 -1 T x , T y 

Tab. 2.2: The universal C 2 symmetry character table. This symmetry is generally a symmetry 

reduction, due to a main symmetry plane being broken, usually originating from a C 2v or a C 2h 

symmetry. 

The tables contain three main areas: the names of each irreducible representation (the 

letters in the far-left column), the symmetry operations and the resulting character of the 

orbitals, symmetric or anti-symmetric (consisting of the bulk of the tables), and the polarization 

of the characters (far-right column). The irreducible representations are known as 

Mulliken symbols and can be directly derived as traces incorporating all symmetry operators 

[46]. The characters A and B, found in all the tables, are one-dimensional and differ 

only when being acted on by the principle rotational operation, C 2 , in its symmetric/antisymmetric 

character. The C 2h orbital subscripts g and u - originating from the German 

words gerade and ungerade meaning even and odd, respectively - refer to the symmetry with 

respect to inversion. An inversion operation, where g is symmetric, results from a s- and 

d-orbital transformation, p- and f-orbitals produce an anti-symmetric u; more explicitly an 

inversion creates a transformation into minus itself. Lastly, the subscripts for C 2v , 1 and 2, 

refer to the orbital character being symmetric or anti-symmetric with respect to a reflection 

through the (xz)-plane. Likewise, C s orbital superscripts ’ and ” denote the same respective 

orbital character, however with respect to the horizontal plane. 

C s I σ h Polarization 

A’ 1 1 T x , T y 

A” 1 -1 T z 

Tab. 2.3: The universal C s symmetry character table. This symmetry is generally a symmetry 

reduction, due to a main reflection symmetry plane being broken from a C 2v symmetry.

2.5 Hartree-Fock Model 22 

C 2h I C 2 i σ h Polarization 

A g 1 1 1 1 

B g 1 -1 1 -1 

A u 1 1 -1 -1 T z 

B u 1 -1 -1 1 T x , T y 

Tab. 2.4: The universal C 2h symmetry character table. The horizontal plane of symmetry makes 

for the identical permission of x and y detection. Where no detectable transmission is listed, the 

transition is considered forbidden. 

C 2v I C 2 σ v (xz) σ v(yz) ′ Polarization 

A 1 1 1 1 1 T z 

A 2 1 1 -1 -1 

B 1 1 -1 1 -1 T x 

B 2 1 -1 -1 1 T y 

Tab. 2.5: The universal C 2v character symmetry table. 

z-excitations, posses individual selection rules. 

Here all three directions, x-, y-, and 

2.5 Hartree-Fock Model 

Theoretical quantum-chemical calculations combined with the measured PES data are a very 

powerful tool in experimental analysis. Not only are the binding energy calculations helpful 

in providing a direct correlation to the PE spectra, but they are also beneficial for determining 

the symmetries associated with each energy level. In simulating three sexithiophene 

conformers, each manipulated to possess a specific symmetry (see Sec. 2.4), total molecular 

potential energies and individual molecular orbital binding energies can be calculated. It is 

the symmetries of the latter that is necessary for the application of group theory. 

There are a variety of calculation models in use today; the most of which are tailored to 

be advantageous for (only) certain situations searching for very particular variables. For our 

purposes of optimizing constructed molecules (i.e. parameters such as bond distances, bond 

angles and dihedral angles), the Hartree-Fock method was implemented, in which quantum 

mechanics describes the molecular geometry in terms of minimum energy (optimal) arrangements 

[49] of the electronic and nuclear charges 17 . We simplify this process by involving 

17 Here we consider a general Schrödinger equation, ĤΨ=EΨ, where Ψ is a many-electron wavefunction 

and the Hamiltonian, Ĥ, is expressed by: 

Ĥ = −h2 

nuclei 

∑ 

8π 2 

A 

1 

M A 

▽ 2 A− −h2 

electrons 

∑ 

8mπ 2 

a 

nuclei 

∑ 

▽ 2 A−e 2 

A 

electrons 

∑ 

a 

nuclei 

Z A 

∑ 

+e 2 

r Aa 

A> 

nuclei 

∑ 

B 

Z A Z B 

R AB 

electrons 

∑ 

+e 2 

a> 

electrons 

∑ 

b 

1 

r ab 

(2.10)


two approximations, the first of which is the Hartree-Fock approximation. 

This involves 

replacing the many-electron wave function of the molecular system by a product of 

one-electron wavefunctions (spin orbitals). These substitutes are known as Hartree-Fock or 

single-determinant wavefunctions. Each spin orbital incorporates a space part (ψ) - function 

of the coordinates of a single electron (molecular orbital) - multiplied by a ± 1 2 

part (either α or β, respectively). The consequential set of differential equations is then 

interpreted by applying the other approximation, the Linear Combination of Atomic 

Orbitals (LCAO) approximation. As the name implies, this approximation uses solutions 

for the hydrogen atom to produce one-electron solutions for many-electron molecules. 

Essentially, albeit over-simplified, this stems from the argument that since molecules consist 

of atoms, solutions to molecules would also be comprised of atomic solutions. The employed 

molecular orbitals are expressed as a set of linear combinations (basis set) forming the basis 

functions (φ) 

ψ i = 

basisfunctions ∑ 

µ 

spin 

c iµ φ µ (2.11) 

The LCAO expands equation 2.11, within which φ µ is referred to as an atomic orbital since 

it describes the behavior of an electron that is generally centered around the position of the 

nucleus (”center of the atom”), and c is the atomic orbital coefficient. 

The combination of these two approximations, when applied to the electronic Schrödinger 

equation, yields the Roothaan-Hall equations [49]: 

basisfunctions ∑ 

ν 

(F µν − ε i S µν )c iµ = 0 (2.12) 

Here, ε is the energy of the orbital, S is the overlap matrix 18 (the interaction of the basis 

functions), and F is the Fock matrix (analogous to the Hamiltonian). 

The results from 

equation 2.12 are collectively termed Hartree-Fock, generally they belong to the class of ab 

initio models. 

Then applying the Born-Oppenheimer Approximation - that the nuclei are fixed in space within the molecular 

system - we simplify the above equation by reducing the nuclear kinetic energy (1 st term) to zero and 

the nuclear-nuclear Coulomb (4 th term) to a constant. This BO Schrödinger, also known as the ”electronic” 

Schrödinger equation, is the starting point for almost all quantum-chemical calculations. 

18 The atomic orbital coefficients are required, by symmetry, to be equal. Thus normalizing ψ is as 

follows [48]: 

∫ 

∫ 

∫ 

1 = ψ ∗ ψ dτ = (c ∗ aa ∗ ± c ∗ ab ∗ )(c a a ± c a b)dτ = 2|c a | 2 ± 2|c a | 2 a ∗ bdτ (2.13) 

Now a and b are normalized and real, resulting in the overlap integral; S ab = ∫ a ∗ b dτ.


Fig. 2.10: Graphical representation of the bonding (φ g ) and anti-bonding (φ u ) process within the 

hydrogen-molecule ion, H + 2 . The systems’ two protons are labelled a and b. 

The Hartree-Fock (HF) model employs a basis set that chooses Gaussian-type functions 

that are very similar to the solutions to the hydrogen atom [49]. To see the pivotal position 

of the LCAO approximation within the HF model, let’s first examine the exact solution for 

the hydrogen-molecule ion, H + 2 . In this case two identical 1s wavefunctions, φ a and φ b , each 

centered around proton-nuclei a and b, come together to form one wavefunction (ψ), as shown 

graphically in Figure 2.10 [20]. The resulting ψ g and ψ u represent the even and uneven mixing 

of the φ’s. Although this is a trivial fact, it is important in our studies of binding energy 

calculations that the reader be reminded of the fundamental energy differences existing 

between even (bonding) and odd (anti-bonding) wavefunctions. The higher stability found 

in bonding wavefunctions lowers its total energy level to below that of the independent 

φ’s. This is due to the larger probability of charge occupying the central region where it 

will enjoy the attractive potentials from both protons. The node in the middle of the antibonding 

wavefunction clearly destroys such chances, and instead distributes the charge more 

toward the outer edges of the molecule, where the electron feels only one nucleus [48]. The 

exact energy splitting is dependent upon the distance between the two nuclei, R ab , as shown 

in the graph in Figure 2.11 [20]. 

However, here the experiments were conducted on a molecule containing sulfur and carbon 

atoms in addition to hydrogen atoms. So the atomic orbitals (φ) that are being combined 

within the LCAO approximation consist of wavefunctions from the 2s (φ s ) and 2p (φ p ) subshells 

19 . The φ p function possesses three asymmetrical parts, each aligning itself along the 

19 Just to remind the reader, the s-wavefuntions are symmetric with respect to the nucleus whereas p-


Fig. 2.11: Graph showing the dependence of the bonding energy, associated with bonding character, 

as a function of the distance between nuclei, R ab . 

three principle axes, x, y and z, where the nucleus is positioned at the origin. 

combination, of φ s and φ px for example, would lead to the new wavefunctions 20 

A linear 

ψ + = φ s + φ px and ψ − = φ s − φ px (2.14) 

A graphical view of these equations is presented in Figure 2.12. In this picture it is clear 

to see that by combining the φ s (dashed line) with φ px (dash-dotted line), the resulting φ + 

wavefuntion (MO) has shifted its center of charge as compared to the s-function. In other 

words, the electronic motion is not centered around the nucleus. The same principle of the 

LCAO approximation expressed in Figure 2.12 is represented as a pictorial in Figure 2.13 

of the atomic orbitals combination for pd (top) and sp (bottom) hybridization (the latter 

hybridization is exactly that plotted in Figure 2.12). Here we see the initial atomic orbital 

(φ i ) being added to the λ of the next subshell to show that hybridization (polarized) effects 

are still encountered, even when factoring in empty subshells. By taking this polarization into 

account, and likewise the dissimilar atomic orbital coefficients (c) that vary with different 

atoms, provides for a more accurate description of the bonding within any molecular system. 

Applied in the present study was the well-accepted polarization basis set 6-31G(d,p). 

This basis encompasses the aforementioned hybridization, up to the d-subshell, as well as introducing 

a split-valence parameter. This parameter essentially includes two separate terms 

to describe the inner-shell and valence electron distributions. The inner shell (σ) electrons 

are all symmetric 21 , and within the 6-31G(d,p) can be grouped together and represented by 

wavefunctions are anti-symmetric. These same symmetry arguments can be applied to molecular wavefunctions, 

which are then known as σ and π, respectively. 

20 This assumption is valid when the 2s and 2p states are degenerate, as is the case in a C-H bond.


Fig. 2.12: In this picture it is clear to see that by combining the φ s (dashed line) with φ px 

(dash-dotted line) that the resulting φ + wavefuntion has shifted its center of charge as compared 

to the s-function. 

six Gaussian curves. The valence orbitals are then split into two functions containing one 

and three Gaussian curves. 

Fig. 2.13: A cartoon showing the polarization effects resulting from including the next subshell 

within the confines of LCAO approximation. 

21 Although the p-wavefunction is antisymmetric, in -plane p x and p y orbitals combine to form symmetric 

wavefunctions. These p orbitals are then also termed ”tight” σ-bonded, to distinguish them from their 

p z -orbital counterparts - they are called ”loose” π-bonded.

Chapter 3 

Experimental 

This section provides a detailed overview of the experimental design and techniques used 

for obtaining the presented data. The experimental setup is separated here into its two 

main components: the MBI-BESSY beamline (Sec. 3.1) providing the synchrotron radiation, 

and the ultra-high vacuum surface end station (Sec. 3.2) wherein the photoemission 

measurements were taken. Details pertaining to the additional (synchronized) laser systems 

available at the MBI-BESSY application lab can be found in Sec. 6. The preparation of 

our sample will be addressed along with a discussion of our methods for substrate and film 

characterization (Sec. 3.3). 

3.1 MBI-BESSY Beamline 

I have already presented to the reader an introductory background to synchrotron radiation 

(Sec. 2.2) and here I will continue the discussion by detailing the particular beamline at 

which all the presented experiments were conducted. A schematic of the MBI-beamline is 

shown in Figure 3.1. The scheme begins with the synchrotron light being released from 

the radiation source, the U125-undulator. Here 125 denotes the periodic length in mm; 

there are 32 periods, and the total overall length of the device is 4m. Spanning three 

harmonics, this undulator is capable of delivering photon energies ranging over 15 - 600 eV. 

The monochromator of this beamline is optimized for a 20-160 eV photon energy range. 

The light from the undulator is deflected by a torroidal (switching) mirror into the MBI 

beamline (U125/2) and focused on the entrance slit of the grazing incidence monochromator 

with an object to image ratio of 10:1 [50]. The spherical grating within the monochromator

3.1 MBI-BESSY Beamline 28 

U125 

top view 

wall 

toroidal 

mirror 

 

plane 

mirror 

entrance 

slit 

spherical 

gratings 

G1,2 

toroidal 

refoc. mirror 

exit slit 

water 

apparatus 

surface 

apparatus 

side view 

surface 

or liquid 

Floor 

[m] 

0 

17.0 18.7 20.4 28.0 30.0 32.0...34.0 

Fig. 3.1: Schematic of the Max-Born-Institute undulator (U125) beamline at BESSY II, top and 

side view. Here the monochromator, between the entrance and exit slits, encompasses both the 

plane mirror and spherical gratings, see text for details. The latter toroidal refocusing mirror 

allows for the light to be directed into one of two end stations, the UHV surface chambers or a 

liquid micro-jet apparatus. 

(detailed in a moment) focuses the light, then passing it through the exit slit into the refocusing 

chamber. Apertures are affixed at both the entrance and exit of the monochromator 

to limit the beam profile. Inside the refocusing chamber are two interchangeable toroidal 

mirrors, which control the final focal size of the radiation and direct it into one of two end 

stations, a UHV surface apparatus and a water microjet apparatus. Any section of the 

beamline has a vacuum of better than 5·10 −9 mbar. 

The monochromator, making up about 10m of the entire beamline, serves two specific 

purposes. Although the aforementioned insertion devices monochromatize the incoming 

synchrotron light fairly well its spectral width is still insufficient for PES. This width is 

roughly determined by ∆E = E ph /n, the photon energy divided by the number of periods. 

For our experimental energies and n being 32, the spectral width is in the range of ca. 1.5-2.0 

eV. 

Further monochromatization is accomplished by using a reflective, rotatable grating (1666 

lines/mm) to tune the emitted spectrum to a particular band pass of wavelengths (in the

3.1 MBI-BESSY Beamline 29 

EUV range). This involves the grating equation 

kN g λ = (sin θ i + sin θ d ) (3.1) 

where θ i,d are the angles of incidence and diffraction, respectively, k is the integer specifying 

the diffraction order, and N g is the number of lines per millimeter on the grating [26]. We 

control the exact excitation energy in diffracting the light at different angles, thus allowing 

only certain wavelengths to pass through the monochromator. For the present monochromator, 

designed for rather steep incidence angles, EUV radiation in the 20-160 eV photon 

energy range is obtained. The typical photon flux is about 4x10 12 /s per 0.1A (electron) ring 

current, and the energy resolution, E/∆E, is better than 6000 near 100 eV photon energy. 

The focal size of the synchrotron light at the sample is ca. 400 µm x 100 µm. 

A detailed schematic of the monochromator optics is illustrated in Figure 2b. This spherical 

grating monochromator (SGM) is based on the variable included angle (VIA) principle - 

a plane mirror (PM) can be translated and simultaneously rotated to ensure that the central 

ray always hits the center of the spherical grating (SG), which can also be rotated about its 

center. The main advantage of this optics system is that the deflection angle 2Q is a free 

parameter, hence the slits always remain at constant position, and the path lengths within 

the monochromator are kept almost constant over the entire energy range. All mirrors and 

gratings are of gold-coated silicon, except for the torroidal-switching mirror, which is of 

gold-covered Zerodur. 

O 

SG 

2θ 

2θ 

B 

A 

SR 

Fig. 3.2: Principle of the monochromator optics at the undulator (U125) MBI-beamline at BESSY 

II. A and B represent the extreme positions of the plane, rotatable gold-coated mirror. In both 

cases would the synchrotron light be deflected (by 2Θ) on to point O, which is the center of the 

spherical grating.

3.2 UHV Apparatus 30 

QMS & UHV 

evaporators 

LEED 

sublimation 

chamber 

preparation 

chamber 

load 

lock 

ion sputter gun 

transfer to 

wobble stick 

synchronized 

laser 

180 o Al/Mg-Kα 

undulator 

radiation 

20-180 eV 

HeI 

discharge lamp rotatable 

analyzer 

. 

analysis chamber 

bellow 

photoemission 

position 

manipulator 

preparation 

position 

Fig. 3.3: Schematic of the UHV surface apparatus end station at undulator (U125) MBI-BESSY 

beamline. The preparation and sublimation chambers are all located along one of the manipulator’s 

principle axis (shown here); the Load Lock chamber can also be accessed from this position. 

Connecting the two sides of the apparatus by a bellow, the manipulator can be moved to the 

photoemission position where it is inserted into the rotatable analysis chamber. The two radiation 

sources coming in from adjacent windows (far left of schematic) represent the synchrotron and 

laser light. 

3.2 UHV Apparatus 

The sample preparation and photoemission measurements were performed in a multi-chamber 

ultrahigh vacuum apparatus with a base pressure on the order of 2·10 −10 mbar. To initially 

obtain this pressure the entire apparatus is heated for a minimum of 24h at ca.150 ◦ C; the low 

pressure is maintained by using several pumps (ion getter and turbo pumps). A schematic 

of the system is given in Figure 3.3. The interconnected chambers are the load-lock, preparation, 

sublimation, and analysis chambers (see labels). Each chamber can be individually 

pumped and separated through a series of valves. 

The 10 mm diameter substrate, mounted on a thin sapphire plate, is initially inserted 

(from air) into the load lock, which has a typical working pressure in the low 10 −8 mbar.


Up to three specimens awaiting preparation can first be stored on a magnetic transfer rod, 

which brings the sample into the preparation chamber. With the aid of a wobble stick, it 

is then affixed onto the main manipulator. Thermocouple contacts fixed at the rear of the 

sample are attached to their counter contacts on the sapphire mount. There are two extra 

contacts available that can be either used for resistive heating or sample biasing. In addition 

to resistive heating there is a possibility for electron impact heating of the sample up to 

1200 ◦ C propagated by a tungsten filament located behind the sample, mounted separately 

on the manipulator. 

The preparation chamber is equipped with standard surface science tools including: a 

4-grid LEED/AES system (ErLEED 3000D, Vacuum Science Instruments) an IS 2000 ion 

sputter gun (Vacuum Science Instruments), and a quadropole mass spectrometer (Balzers, 

QMS 421) that can be used for the analysis of atoms/molecules with a maximum mass of 

ca. 2048 amu. Extending from the preparation chamber along the same axis is the actual 

deposition (sublimation) chamber containing a Knudsen cell providing stable deposition conditions, 

in terms of a well-defined temperature and deposition rate. Typically, the Knudsen 

cell is out-gassed for 10h before an actual 6T deposition. 

To monitor the deposition rate a quartz microbalance (STM-100/MF, Sycon Instruments) 

is installed in the sublimation chamber and can be placed at the sample deposition position 

using a retractable bellow. The evaporation rate is checked for stabilization before the 

deposition and again after to evaluate for any fluctuation and account for it for assigning 

film thickness. Lastly, several windows and user ports of different sizes and at various 

positions are available throughout all of the aforementioned chambers. 

Connecting the preparation chamber(s) with the main part of the UHV apparatus (analysis 

chamber), where the PES measurements occur, is a horizontal manipulator arm and 

flexible bellow. A photo of the full apparatus, with the manipulator arm in the extreme position 

appropriate for preparation is presented in Figure 3.4. The translation of the sample 

along the main axis of the chambers (which is the manipulator main-axis) is realized with 

the aid of an electrical motor. The perpendicular, vertical, and horizontal adjustment to the 

manipulator (and hence sample mount itself) is controlled by two µm screws calibrated to 

a fixed reference distance for reproducibility. The sample can be rotated around its surface


Analyzation Chamber 

Hemispherical analyzer 

Preparation 

Chambers 

Water 

micro-jet 

apparatus 

Synchrotron 

Radiation 

Load lock 

Manipulating arm 

Fig. 3.4: Photograph of the MBI-BESSY beamline experimental set-up. Here the manipulator is 

in the preparation position; the synchrotron light (as labeled) is coming out of the page and would 

be directed into either the analyzation chamber or the liquid micro-jet apparatus (far left of the 

picture). 

normal (azimuthal rotation) and tilted up to 90 ◦ . Likewise the manipulator can rotate 360 ◦ 

around its center axis. This versatility in sample positioning allows for measuring PES in 

almost every orientation geometry with respect to the incident synchrotron radiation. In 

addition, can the electron energy analyzer, housed in the main analysis chamber, where all 

spectra were obtained, be rotated around the axis of the synchrotron beam. In fact the entire 

analysis chamber can be rotated ±90 ◦ (see Figure 3.3). This allows us to take full advantage 

of the polarized synchrotron light, and realize the various experimental excitation/detection 

combinations that are necessary for the present work. 

All spectra were obtained in the analysis chamber, which also contains an X-ray source 

(Omicron, DAR 15) with an Al and Mg twin anode (providing Al-K α : 1486 eV and Mg-K α : 

1253 eV photons), typically used for additional sample characterization. Several optical ports 

are available for the laser beam. Both laser and synchrotron pulses can be simultaneously 

detected by a fast EUV photodiode (IRD) mounted on a separate manipulator. Several CCD

3.3 Materials and Film Preparation 33 

cameras may be used to monitor the position of the sample, as well as the actual irradiated 

spot, which may be observed through fluorescence from a suitably coated metal plate next 

to the sample. The electron energy analyzer (Omicron EA 125 U5) has a 125 mm radius and 

is equipped with a 5-channeltron detector. Photoelectrons were typically detected within 

a ± 1 ◦ sample take-off, and the Fixed Analyzer Transmission (FAT) scan mode was used, 

usually using 10 eV pass energy. This corresponds to about 100 meV resolution. 

Lastly, the majority of the experiments were obtained for BESSY multi-bunch operation 

(500 MHz), which corresponds to a temporal spacing of synchrotron pulses of 2ns. As will 

be discussed in section Sec. 4.2, sharp undistorted photoemission spectral features of the 

films can only be obtained in the presence of a charge-compensating laser. We accomplished 

the removal of the surface charge with the simultaneous irradiation from a high-repetition 

rate Ti:sapphire laser system (83 MHz corresponding to 12 ns inter pulse spacing). Even 

though there is only one laser pulse per every six synchrotron pulses, it is sufficient for our 

needs of charge compensation; laser and synchrotron pulses don’t need to be synchronized 

for that. The same laser system can be used for single bunch operation, since the electron 

density contained within any given electron single-bunch is only about a factor 10-20 higher. 

However, for single bunch measurements we typically used a different laser system, Nd:YVO 4 

operating at an effective 1.25 MHz repetition rate; this coincides with the BESSY single 

bunch frequency, corresponding to the round trip time of a single electron bunch of 800 ns. 

It should be noticed that the low-repetition Nd:YVO 4 laser is insufficient to compensate for 

the sample charging generated in multi-bunch. Both laser systems are further detailed, with 

a description of their parameters and characteristic temporal pulse structure, in Sec. 6. 

3.3 Materials and Film Preparation 

Sexithiophene (6T) powder has been purchased from Syncom BV and used here without 

any further purification (other than 10h outgassing in UHV at 220-230 ◦ C). Films of 6T 

were prepared by sublimation in UHV on single-crystal substrates, for mostly on Au(110). 

The organic material was evaporated from a water-cooled Knudsen cell at about a 10 cm 

distance from the sample, with the latter being at about 40-50 ◦ C temperature. Deposition 

was typically performed at 250 ◦ C temperature of the cell, yielding ca. 0.2Å/s deposition

3.3 Materials and Film Preparation 34 

rate as measured by a quartz micro balance (set for 1.55 g/cm 3 density of 6T [51]). The base 

pressure in the chamber under deposition conditions was better than 8x10 −10 mbar; this was 

achieved for well-outgassed material only. Pulling back the sample, the quartz microbalance 

could be mounted exactly at the sample’s deposition position; the rate was checked both 

before and after deposition. All photoemission measurements from films reported here were 

performed directly after preparation. Throughout this work film thickness given here refers 

to the deposited mass thickness measured by the quartz microbalance. 

The Au(110) and Au(111) single crystals, 10 mm diameter and 1.0 mm thick, were 

purchased from Mateck. The exact orientation of the crystal (roughness) was < 1 ◦ . A clean 

gold surface was obtained by several cycles of 800 eV argon ion bombardment at ca. 300 ◦ C, 

in between which the sample was annealed to 720 ◦ C for about 30s. Sample cleanliness was 

confirmed by the characteristic sharp LEED patterns.

Chapter 4 

Results and Discussion 

4.1 6T Film Characterization: Growth pattern, Thickness, 

and Morphology 

Parallel to the valence band photoemission measurements the 6T films are characterized 

by Low Energy Electron Diffraction (LEED), Atomic Force Microscopy (AFM), and X-ray 

Photoelectron Spectroscopy (XPS). The former method was particularly useful to confirm 

the initial ordered growth of the 6T molecules and provided estimates of surface coverage 

as inferred through deposition rate (at a given evaporation temperature). This comparison 

is based on a LEED study reported in the literature. The morphology of thick films has 

been investigated by AFM. Complementary XPS measurements, using a laboratory source, 

provided additional information on the 6T growth patterns, over a large range of thickness. 

Furthermore, a comparison between gold substrates, Au(110) vs Au(111), was conducted 

using all three techniques. 

4.1.1 Low Energy Electron Spectroscopy (LEED) 

The measured LEED images are compared to studies reported in the literature [39] to corroborate 

that the Au(110) substrate indeed induces an initial, preferred 6T film growth. LEED 

images were recorded for the clean reconstructed (1x2) Au(110) surface 1 and for 6T deposition 

in systematic steps of 0.5 Å mass thickness, up to 8.0 Å total deposition, as monitored 

by a quartz micro-balance. Figure 4.1 shows LEED images, using 50 eV electron energy, of 

1 Clean Au(110) reconstructs to a (1x2) structure, in which alternate [110] rows of atoms are removed 

yielding stable Au(111) facets (trough-walls of the missing row) with a periodicity of ca. 8.1 Å [52], [53].

4.1 6T Film Characterization: Growth pattern, Thickness, and 

Morphology 36 

the clean Au(110) surface (top), for 1.0 Å (center) and 2.0 Å (bottom) 6T deposition. The 

main diffraction maxima are labeled in the figure. The characteristic (1x2) LEED pattern of 

the clean reconstructed Au(110) can be clearly identified. For 1.0 Å 6T deposition a (quasicommensurate) 

(1x3) pattern, slightly streaked along the [001] direction, is observed. Notice 

that no changes in the diffraction pattern appear along the [110] direction. A similar image, 

yet more streaked and less intense, is obtained for 1.5 Å deposition (not shown). For 2.0 Å 

6T deposition most of the LEED reflexes are very faint. This pattern, albeit decreasing in 

intensity, is still observable for depositions as high as 6.0 Å until no diffraction spots can be 

resolved. 

Qualitatively, our measured LEED images are in rather good agreement with comparable 

studies of the identical system reported in the literature. 

We thus briefly outline 

the proposed interpretation of the LEED patterns, which has in fact been corroborated by 

additional scanning tunneling microscopy (STM) and helium atomic scattering (HAS) experiments 

[39], [52]. Without going into deep detail, we attribute this (1x3) structure, observed 

in Figure 4.1 center, to an absorbate-induced reconstruction of the gold atoms. Note that 

the Au(110) substrate is in fact likely to undergo several reconstructions as a function of 

coverage [53], [54]. This has been attributed to the maximization of the contact area between 

6T molecules and the Au(111) trough facets [ref] - as the 6T substrate interaction is believed 

to be stronger than the van der Waals bonding 2 . Here it is useful to refer to the schematic 

shown in Figure 4.2, which illustrates the proposed structural relationship between different 

6T coverages and the Au(110) substrate [ref]. The observed transition from a Au (1x2) to 

(1x3) LEED pattern (see Figure 4.1, top vs center) would be consistent with a widening of 

the missing row to better accommodate the asymmetric 6T molecules, assumed to arrange 

with their long axis along the [110] direction. This changes the Au surface [001] unit vector 

from 8 to 12 Å where the latter corresponds to the 12 Å reconstruction illustrated in the 

top schematic of Figure 4.2. Not all reconstructions suggested, however, are seen in LEED. 

In fact, only the reconstruction in the top figure has been firmly identified through LEED, 

yet the other reconstructions would seem in line with HAS [52] and STM results [39]. The 

2 It is this strong substrate - 6T interaction (> Van der Waals), with a reported energy relaxation difference 

that balances the low energy cost of the Au substrate atoms re-arrangement between a (1x2) and (1x3) 

surface [39], [53].


Morphology 37 

0 Å 

(0, 3/2) 

[001] 

(-1, 1) 

(-1, 1/2) 

(0, 1) (1, 1) 

(0, 1/2) (1, 1/2) 

[110] 

(-1, 0) 

(-1, -1/2) 

(0, -1/2) 

(0, -1) 

(0, -3/2) 

1 Å 

(-1, 1) 

(-1, 2/3) 

(-1, 1/3) 

(-1, 0) 

(-1, -2/3) 

(0, 4/3) 

(0, 1) 

(0, 2/3) 

(0, -1/3) 

(0, -2/3) 

(0, -1) 

(0, -4/3) 

[001] 

[110] 

2 Å 

[001] 

(0, 1) 

[110] 

(-1,0) 

(0, -1) 

Fig. 4.1: LEED images for clean Au(110) (top), 1 Å (middle), and 2 Å 6T coverage/Au(110). 

The (1x2) clean gold image changes to a (1x3) pattern upon the first 6T deposition (see labeled 

peak maxima). All images were taken using 50 eV electron energy.


Morphology 38 

[110] 

12 Å 

16 Å 

20 Å 

Fig. 4.2: The sketch shows an in-plane view (see directional arrows) of the 6T deposition in 

which the 6T molecules (shaded ovals) are depicted as lying) within the troughs (with their long 

axis parallel to them) but with a possible tilt in favor of the maximum contact (little arrow) area 

provided by the trough wall, the Au(111) microfacet. The open circles represent gold atoms and 

are shown after the first 6T deposition in their re-constructed (1x3) array. The 12, 16, and 20 Å 

values are the trough-trough width as discerned using STM and HAS measurements; note that all 

values are larger than the observed ca. 8.1 Å trough-trough distance for clean gold [39]. 

[001] 

reasoning behind LEED’s failure in tracing any other reconstructions - not here and not 

for the reported images- is unclear. It may be, however, that this information is hidden 

by the streaks in the LEED images, which might result from the coexistence of small-sized 

domains of ordered structure but in various directions, other defects, and a combination of 

substrate re-constructions (1x2) and (1x3). Notice that the observed changes in the LEED 

patterns are argued to directly reflect the substrate reconstruction. In other words, the reflexes 

originate from the Au atoms rather than from the organic overlayer, which is here 

attributed to the fact that electrons are more strongly scattered by the high-Z Au atoms at 

such electron energy [39]. The electron mean free path near 50 eV is about 3-4 layers (see 

Figure 2.2). However, to reiterate what is important for our purposes is that undeniably, if 

our 6T molecules did not well-align themselves with respect to the Au atoms, we would have 

no change in the diffraction pattern, except for a faster rising background intensity, which 

simply would drown out the Au peaks. 

A tentative assignment of coverage to our measured LEED patterns can be attempted by 

again comparing with literature. The center image in Figure 4.1, 1.0 Å deposition, closely 

resembles the reported LEED pattern corresponding to the structure in Figure 4.2(top), 

assigned to 0.7 ML. Hence, 1 Å mass deposition (where we have assumed the 6T bulk 

density, 1.55 g/cm 3 , for setting the micro balance) would correspond to about only 40-50 %


Morphology 39 

of the bulk surface coverage; this is a rough number simply estimated from Figure 4.2. Bulk 

coverage could only be claimed for the bottom schematic in Figure 4.2. Then, for our crude 

(internal) reference, we attribute 2-2.5 Å 6T deposition to ca. 1 ML 6T effective coverage. 

This is about the situation illustrated in the bottom sketch in Figure 4.2. 

We conclude this section with a comparison of LEED images of 6T/Au(110) vs 6T/Au(111). 

Figure 4.3 displays the respective images with the amount of deposited 6T indicated; electron 

energy was 80 eV. The top two frames evident the respective clean gold surfaces. The 

LEED images show the characteristic sharp (1x2) pattern for the reconstructed Au(110) 

surface (left), and a hexagonal pattern for the Au(111) surface (right). The first noticeable 

observation is that 6T deposition on Au(111) (e-f) only causes a fading of the gold signal, 

without any sign of reconstruction (or any quasi-commensurate superstructure) as no extra 

diffraction peaks arise. Hence the 6T molecules are concluded to nucleate at random. 

Secondly, LEED spots fully disappear for the 6T/Au(111) at considerably lower deposition. 

This directly suggests a denser 6T packing in the monolayer regime owing to the less structural 

confinement (e.g. the corrugated Au(110) step) in accommodating 6T molecules. This 

is, however, on the expense of structural order. Specifically, in Figure 4.3(f), corresponding 

to 1.5 Å deposition, signal from the Au(111) surface can no longer be observed, while it is 

visible on Au(110) for this, and thicker, coverage. 

This comparative LEED study indicates the growth of ordered 6T films on the Au(110) 

substrate for sufficiently low coverage. Notice, however, that no LEED pattern could be observed 

for thickness > ca. 4-5 ML, even though we have attempted to push the limit to higher 

coverage. This was done by varying both the substrate temperature (room temperature to 

150 ◦ C) and the deposition rate (0.05 - 0.5 Å/s). It is important to recall here that 6T, within 

the initial monolayer, resolutely interacts with the gold, probably trying to gain maximum 

surface contact. Whereas in the succeeding monolayers interaction occurs intermolecularly, 

only with their organic neighbors, eventually forming a bulk crystal structure [Ref, Marks]. 

6T does, in fact, preserve a memory of the surface structure [52] in subsequently grown 

mulitlayer films. Later we will briefly discuss the nature of the 6T-to-substrate interaction 

in terms of electron binding energies between submonolayer and multilayer 6T films. We 

conducted our photoemission experiments on sufficiently thick films, too thick for LEED


Morphology 40 

[001] 

0 Å/Au(110) 

0 Å/Au(111) 

[110] 

a) 

d) 

1 Å 6T 

Au(110) 

1 Å 6T 

Au(111) 

b) 

e) 

1.5 Å 6T 

Au(110) 

1.5 Å 6T 

Au(111) 

c) 

f) 

Fig. 4.3: LEED images obtained for 80 eV electron energies for clean Au(110) and Au(111), left 

and right columns, and subsequent low coverage of 6T molecules, 1 and 1.5 Å respectively. We see 

an appearance of new diffraction peaks, along with some streaking, along the [001] row as evidence 

of our substrates reconstruction. Whereas the LEED spots for its Au(111) hexagonal counterparts 

remain fixed and fade faster.


Morphology 41 

analysis. Instead we used XPS and AFM, which will be elaborated on below, to examine if 

the 6T molecules remained highly structured in the thicker films. 

4.1.2 Atomic Force Microscopy (AFM) 

AFM is used in the present study to characterize thick 6T-film morphology. The AFM images 

confirm that the well-ordered growth of 6T molecules on Au(110), as seen for low coverage, 

continues in the growth of three-dimensional features (islands) even after many hundreds 

of layers [35]. Our findings are then in line with a Stranski-Krastanov (SK), or layer-andisland 

growth mode, which has been reported earlier [30]. This implies an initial monolayer 

growth (one or more) that eventually breaks down and is followed by the nucleation and 

growth of 3-dimensional islands on top of the complete layer(s). For analogous SK films of 

6T/Au(111), no directional preference is exhibited of the islands within the films. 

Figure 4.4 shows AFM images of (a) 500 Å 6T/Au(110), (b) 1000 Å 6T/Au(110), and 

(c) for comparison 500 Å 6T/Au(111). All topographical AFM images were obtained exsitu 

using tapping mode; tip spring constant (C) ranging between 27-89 N/m, and recorded 

at 1.25Hz, 512 lines per scan. 

For statistical purposes several scans were performed of 

different sample areas. Chains of oriented interconnected islands can be clearly identified 

for the 500 Å 6T/Au(110) system (top), i.e. there is no homogeneous film growth for this 

large thickness. For 1000 Å 6T/Au(110) (middle) we see larger microcrystallites (quantified 

below) still exhibiting a retained order. 

Upon rotating either of these samples, with respect to the tip scanning direction, the 

AFM image rotates by the same degree and remains intact with its distinct directional 

disposition. Rather similar morphology has been observed for 6T films grown on substrates 

with preferred absorption sites, such as potassium acid phthalate (KAP) (001) crystals [1], on 

films that were a little thinner, ca 200 Å. The AFM is not capable of inferring the orientation 

of the individual 6T molecules within the islands, but we could determine that the preferred 

direction of the microcrystallite chains is along the underlying [110] (Au-)crystal axis. We 

will furthermore argue, based on PES measurements, that the molecular long axis also 

coincides with the [110] direction of the Au(110) substrate. 

The image of 6T/Au(111) (Figure 4.4, bottom) appears to be obscured by tip-induced


Morphology 42 

(a) 500 Å 6T/Au(110) 

10 µm 

(b) 1000 Å 6T/Au(110) 

10 µm 

(c) 500 Å 6T/Au(111) 

7.5 µm 

10.0 µm 

Fig. 4.4: AFM images of (a) 500 Å 6T on Au(110), (b) 1000 Å 6T on Au(110), and (c) 500 Å 

6T on Au(111), top views. Images were made in ex-situ in tapping mode at 1.25 Hz. A preferred 

island orientation is easily identifiable in the top two frames, whereas the bottom image shows a 

random nucleation of 6T microcrystallines. For clarity, lines mark the LEED reflexes in the bottom 

images.


Morphology 43 

artifacts; hence the shape and dimensions of the islands cannot be accessed. Despite this 

fact, however, if the islands possessed a preferred orientation it would still be observable 

in AFM. This is not the case here since we see a random adsorption of island structures. 

Likewise, the random island film morphology, as seen on Au(111), has been similarly reported 

for 6T deposited on other unstructured substrates such as mica [35] and silica [1]. 

is also consistent with the structure discussed in the previous section (LEED) - where we 

concluded that the ordered 6T film growth is only obtained by a substrate with an adsorption 

(geometric) restraint, which defines specific nucleation sites, e.g. the missing row structure 

of the Au(110). 

This 

nm 

-100 0 100 

0 2.00 4.00 6.00 8.00 nm 

nm 

-250 0 -250 

0 2.50 5.00 7.50 10.0 

nm 

Fig. 4.5: Cross section cuts of the AFM images from Figure 4.4. This illuminate the profile of the 

surface islands, specifically smaller independent (well-defined) towers on the Au(110) substrate. 

Cuts were taken along the orientated rows for the images on Au(110). 

A profile of the Au(110) images in Figure 4.4, to quantitatively assess the islands’ size, is 

shown in Figure 4.5. Numerous profiles were created along different regions of the films, thus 

the exact placement of the profiles is somewhat arbitrary. The arrows are guidelines for the 

reader as to height dimensions. Measured dimensions are summarized in Table 1. Average 

heights are ca. 30 nm, and widths are 250 nm for the relatively compact microcrystallites


Morphology 44 

film thickness (Å)/substrate Average height Average width 

500 Å 6T/Au(110) 35 nm 250 nm 

1000 Å 6T/Au(110) 80 nm 500 nm 

Tab. 4.1: A summary of the measured dimensions for the cross section profiles. The values are 

calculated over several sample areas, representing an average value over all all images. The section 

cuts are aligned along the preferred rows for the Au(110) samples and across high tower-density 

film spots for the Au(111). Notice that even ca. doubling the film thickness on Au(110) the islands 

remain noticeably narrow as the Au(111) film. 

imaged for 500 Å 6T on Au(110); the islands tend to be rather separated from each other. 

For doubled 6T deposition, 1000 Å 6T, we continue to observe inter-connected towers, which 

remain relatively distinguishable, increasing rather in height (ca. 80nm) than in width (ca. 

500nm). This specific growth mode for 6T/Au(110) is shown in the next section as also 

reflected in the XP spectra. 

4.1.3 X-Ray Photoelectron Spectroscopy (XPS) 

We have since characterized our 6T films’ structure and morphology, for low and high coverage, 

respectively, using two different (surface) methods, LEED and AFM. In this section 

we show that our conclusions as to the films’ structure are also reflected in the XP spectra. 

Figure 4.6 displays XP spectra, in the 200-400 eV binding energy range, recorded for 

several coverages of 6T on Au(110) using 1486 eV (Al-K α ) photon energy. The coverage 

indicated, 0.5-500 Å, refers to the deposited 6T (mass) thickness as determined by a quartz 

micro balance, and discussed in Sec. 4.1.1. Coverages were increased by on-top deposition 

on the previous film, and PE spectra were measured in between the depositions. Deposition 

was at ca. 250 ◦ C Knudsen cell temperature, yielding a rate of about 0.2 Å/s. At deposition 

the gold crystal was always at 40-50 ◦ C temperature. For clarity the spectra in Figure 4.6 

are vertically displaced relative to each other. Even though the XP spectra were obtained for 

nominally identical power settings of the X-ray tube, small variations were noticeable from 

run to run, which we accounted for by normalizing the spectra to the constant background 

signal near 370-400 eV electron binding energy. The bottom trace shows the XP spectrum of 

the Au(110) surface with submonolayer 6T coverage, exhibiting the prominent Au4d 3/2 and 

Au4d 5/2 emission lines, as labeled. With increasing coverage the gold signal is attenuated 

while new features, originating from C1s and S2s, appear at 284 and 231 eV, respectively.


Morphology 45 

Au4d 3/2 

Au4d 5/2 

C1s S2s 

Photoemission SIgnal [arb. units] 

8000 

6000 

4000 

2000 

500 Å 

100 Å 

10 Å 

5 Å 

0.5 Å 

-400 -350 -300 -250 

Binding Energy [eV] 

Fig. 4.6: XP spectra for increasing coverage of 6T on Au(110). X-ray source was an Al-K α , 

emitting electrons at 1487 eV. Note that the pronounced Au4d twin peaks are persistent, even 

through thick films (ca. 500 Å). The appearance of the strong C1s and smaller S2s shows the 

deposition of the organic material (see labels). The ratio between the substrate and adsorbate 

signals is a further conformation of the porous morphology of the thin films. 

The substrate signal is still observable in the 500 Å spectrum (upper trace in Figure 4.6), 

which is only possible for a rough film surface, exhibiting deep ridges. Tentatively identifying 

2-2.5 Å deposition with one effective monolayer (Sec. 4.1.1), at 50 Å total 6T deposition 

the Au signal should in fact be suppressed by ca. 90% 3 , provided the film were smooth. 

Hence, Figure 4.6 confirms the high-corrugation (”open”) film morphology arrived at in the 

previous sections. 

Apparently, low 6T-coverage must exist in between the 6T chains of islands, even up to 

>500 Å 6T deposition, explaining the emission contributions at those high coverages. Also 

the initial slow increase of both the C1s and S2s signal, from 5 to 10 Å mass thickness can be 

attributed to island growth, however, preferentially in height growth. Then, at this point, 

3 If growth were layer-by-layer the substrate signal would decay exponentially according to [17]: 

I ad = I 0 exp (−d)/λe (4.1) 

Here I 0 and I ad denote the photoemission intensity of the clean and the covered substrate, and d and λ e are 

the layer thickness and the electron mean free path. For 1150 eV (≈ excitation energy (1486 eV) - binding 

energy (ca. 350 eV)) λ e is about 10 ML (see Figure 2.2). Hence, with λ e ≈ 10 ML ≈ 20 Å 6T deposited, 

one may calculate the film thickness for which the initial substrate signal is attenuated by 90% as follows: 

0.1 = 1.0 exp(-d/2nm). This yields d ≈ 5 nm or ca. 50 Å 6T deposition.


Morphology 46 

7000 

6000 

C1s 

500 Å 6T/Au(111) 

500 Å 6T/Au(110) 

Photoemission Signal [arb. units] 

5000 

4000 

3000 

2000 

Au4d 

S2s 

S2p 

Au4f 

1000 

Source: Al-K α 

0 

-350 -300 -250 -200 -150 -100 -50 0 


Fig. 4.7: XP spectra obtained for hν 1487 eV (Al-K α source) of two thick films 500 Å 6T on 

Au(111) (top) and on Au(110) (bottom). These spectra correspond to the same films studied 

by AFM. Note the similarity between organic features but the differences in the large persistent 

signals arising from the gold features’ Au4d (353.2 eV and 335.1 eV BE) and Au4f (87.6 eV, and 

83.9 eV BE) double peaks on the Au(110) substrate. This illustrates the porous nature of such 

thick films. 

the islands’ height is likely on the order of the electron mean free path. The organic signals 

can thus not further increase, unless the islands start growing laterally. Increasing lateral 

growth is presumably responsible for the steeper signal rise for larger thickness, consistent 

with our AFM images. Notice that also the absence of pronounced steps in the XP spectra of 

organic films, which are characteristic of flat (homogeneous) surfaces, is a further indication 

of a rough surface structure. 

Figure 4.7 compares XP spectra for 500 Å 6T deposited on Au(110) vs Au(111), again 

obtained for 1486 eV photon energy. 

These measurements were performed on the same 

samples that were then afterwards subjected to AFM measurements (yielding the respective 

images shown in Figure 4.4). The figure displays an expanded binding energy range, 60-370 

eV, to particularly include the strong signals from S2p and Au4f 5/2 and Au4f 7/2 peaks at 163 

eV, 87.6 eV, and 83.9 eV, respectively. Notice the gold signal attenuation is considerably 

larger for the Au(111) surface, as would be expected from our AFM and LEED results. 

Specifically, the Au4f signal for the Au(111) system is about half the intensity as compared

4.2 Photoemission Spectra and the Need for Laser Excitation 47 

to the Au(110). 

4.2 Photoemission Spectra and the Need for Laser Excitation 

The main goal of the present work is to investigate the electronic structure of sexithiophene 

molecules within thin films of well-ordered structure. Examining these films is crucial in 

better understanding fundamental film properties, which of course differ from those of the 

sub- to a few monolayers regime (interface) and of the bulk crystal. As we have discussed in 

the previous sections, reasonably ordered films consisting of micro-crystallites, as high as 80 

nm, can be grown on a Au(110) single crystal template. However, the photoemission from the 

gold substrate is particularly persistent due to the inhomogeneous film structure, as we have 

already seen in XP spectra in Sec. 4.1.3. In the valence band region, especially the strong 

Au-5d emission severely masks the photoemission contributions of the organic material; it 

is still detectable up to about 500 Å mass thickness, as is the Au contribution at the Fermi 

level. Hence, in studying the films’ anisotropic photoemission it would be advantageous to 

use even thicker films in order to sufficiently suppress substrate contributions, as the effects 

we are interested in may be small and thereby difficult to distinguish from signal changes due 

to the substrate. Such thick films of about 2000 Å however, tend to suffer from considerable 

sample charging. This leads to spectral broadening as well as peak shifts in the PE spectra, 

which prevents accessing accurate electron binding energies. It would be then impossible to 

detect the small changes in peak positions and intensities that would be intrinsic to the 6T 

film. For the present study, charging is compensated by simultaneous laser light excitation. 

Then the main focus is on (1) obtaining sharp photoemission features from 2000 Å 6T films, 

and (2) interpreting angle-resolved PE spectra by theoretical calculations with respect to 

the molecular orientation, through molecular orbital symmetries. 

4.2.1 6T Spectral Evolution and Assignment Through Coverage 

To begin characterizing our system, Figure 4.8 shows normal emission PE spectra of 6T on 

Au(110) as a function of coverage obtained for grazing photon incidence (Ψ= 83 ◦ fixed), 

using 50 eV photons, with the light polarization vector being perpendicular to the 6T long


60x10 3 

2000 Å 


50 

40 

30 

20 

10 

2000 Å 

500 Å 

100 Å 

20 Å 

5 Å 

Clean Au(110) 

S3s & 

C2s 

D 

S3s & 

C2s 

C 

S3s & 

C2s 

S3p z 

B 

C2p z 

A 

E f 

0 

hν = 50 eV, normal emission 

Au-5d 

-15 -10 -5 0 


Fig. 4.8: Photoemission spectra of 6T on Au(110) as a function of coverage obtained in normal 

emission for 50 eV photon energy at a grazing incidence with the polarization of the light along 

[001]. Clean gold (bottom spectrum) has features arising in the same binding energy region as 6T 

valence band signal, with respect to the Fermi edge (solid black line). The evolution and shifts 

(dotted lines) of the determined organic features (see labels on top spectrum) according to film 

thickness is discussed in the text. 

axis. The purpose of this series of spectra, being obtained without additional laser light, is 

to identify the spectral features, and to follow their evolution (with respect to peak shifts 

and widths) as the 6T film grows. The spectra are normalized with respect to the secondary 

electron signal, and the electron binding energies (BE) are presented relative to the Fermi 

energy. For clarity successive spectra are vertically offset. Peak assignment is only generally 

allocated into large features (A-D) for this graph and will be returned to and refined below. 

Clearly, the 2000 Å trace is greatly shifted to higher binding energies. Hence, to guide the 

reader in spectra comparison the 2000 Å spectrum is reproduced (top, dashed line), redshifted 

by 2 eV, in order to align with the previous 500 Å spectrum. An additional shift of 

only ca. 0.2 eV toward higher binding energies can be noticed in the spectra between the 

5 and 20 Å traces. Since these shifts are of fundamentally different origin, it is useful to 

separate the analysis according to film thickness. 

Clean Gold and Low Coverage (few Monolayer) Regime. The PE spectrum of 

the clean gold substrate (Figure 4.8 bottom) exhibits the Au-5d emissions at 5.9, 3.9, and 3.0 

eV binding energy [55], [56]. Upon low 6T deposition, ca. 5 Å mass thickness (although they


are more defined for ca. 20 Å), the organic features, peaks D and C, emerge near 13.5, 11.5, 

9.0, and 6.5 eV binding energies, respectively (see labels). These peaks are conglomerate 

signals from the S3s, C2s, and S3p electrons; whereas the poorly resolved π-peaks, labeled A 

and B between 1.5 - 4.5 eV, are mainly composed of S3p and C2p contributions [29], [56]. 

In this low-coverage region, specifically between 5 Å and 20 Å mass thickness 4 , corresponding 

to ca. 2 and 8 monolayers (ML), respectively, a shift of 0.2 eV toward higher BE 5 

is observed in the organic peak D. There are three possible sources for this shift that should 

be reviewed: (1) adsorbate-substrate direct chemical bonding (hybridization), (2) electronic 

screening, or (3) a change in the molecule’s conformation. Notice that hybridization (1) 

would be seen in the π-derived features, which for low coverage are, however, masked by 

the strong superimposed Au5d contributions. Although hybridization would be expected to 

occur only between the Au d-electrons and π-electron system, the observed σ-shift at ca. 11 

eV could still be an effect driven by such hybridization. For example, even if the π-features 

show no shift but there is an overall shift in the work function of the sample (inducing an 

identical shift in the σ-features), then the π-orbitals have a different binding energy relative 

to the σ-C and -S peaks [30]. However, again without defined π-spectral features it is impossible 

to unequivocally determine (from this figure) whether or not the observed σ-shift stems 

from an interfacial chemical bond. Chemical interactions between thiophene molecules and 

gold are expected (theoretically [37]) to be weak though, however, experimentally the issue 

is debated [38], [39]. 

Screening (2), another possibility for the blue-shift in peak D, is an effect due to the positive 

hole left behind in the film after the photoemission process. The escaping photoelectron 

will be slowed down as it experiences the Coulomb forces due to this positive charge. As 

a result the electron will be measured at a reduced kinetic energy (final state effect). For 

sufficiently low coverage, electrons from the metal substrate will quickly fill the holes, hence 

this screening effect would be negligible. The ”best” screening is then clearly found in the 

first monolayer, which is in direct contact with the metal [57], [58]. As the film grows thicker 

the filling of holes by free charge carriers is slowed down, and peak shifts (to lower BE) are 

more likely to be observable. 

4 In the following we will simply refer to thickness instead of mass thickness. 

5 Peak contributions from both the monolayer and (sections of) mulitlayers may lead to peak broadening.


Lastly, a structural conformation (3) within the molecules could also lead to a shift in 

binding energy. It has been argued here and shown elsewhere [41] that the 6T molecules assume 

a planar geometric structure upon the initial adsorption, and through a few monolayer 

growth, on Au(110). This structure, though, may be energetically beneficial only directly at 

the interface. As 6T deposition continues, molecules begin to contort as they structurally 

rearrange to a structure at their potential energy minima. Thereby the former planar structure 

is replaced by a molecular conformation with a reduced symmetry, as has been reported 

e.g. for 6T/Ag(111) [36]. In conclusion, we suggest a combination of the three described 

effects as likely to occur in our low coverage system. 

Thick (Multi-Layer) 6T Films. Upon further 6T deposition (ca. 20 - 100 Å) no new 

spectral shifts arise; the only noticeable effect is the increasing attenuation of the substrate 

features. This is reasonable since effects (2) and (3) can occur only in the transition within 

low-coverage regimes, and shift (1) only in the monolayer regime. Only for coverage larger 

than 500 Å is a shift of all features observed. This is attributed to the onset of sample 

charging. The particular charging shift of about 2.0 eV for the 2000 Å 6T film corresponds 

to the specific experimental conditions (e.g. synchrotron flux), which will be detailed further 

in Sec. 4.2.2. 

Up until 500 Å we have received additional Au contributions in our 6T valence PE 

spectra. However, only at a thickness of 2000 Å 6T are the gold features fully quenched, as 

best reflected by the absence of the Fermi level. Thus for 6T/Au(110), thick film conditions 

are the most suitable to studying angle-resolved PE. Notice that the top trace is the PES 

of a pure 6T film; the features are however broadened. Hence, the emerging features at 

2.25 and 4.1 eV, previously coinciding with the Au 5d substrate emission, for the 2000 Å 

6T spectrum are assigned to the organic film’s π-features. Still, the peak assignment in 

Figure 4.8 is referred to rather generally, since features are only poorly resolved. Betterresolved 

spectra will be obtained if we compensate for charging, which is the topic of the 

next section.


4.2.2 Film Charge Compensation in the Presence of Laser Irradiation 

The ejection of an electron by the photoemission process creates an uncompensated positive 

charge (holes) in the organic film. As this positive charge may accumulate in low-conductive 

films, a positive surface potential is produced. Hence, the emerging photoelectrons will be 

slowed down, resulting in a rigid spectral blue-shift, as if the electrons had higher binding 

energies. 

Often this shift is accompanied by spectral broadening, which would typically 

occur for rough surfaces, as in the present case. Then, regions of larger thickness are likely 

to be more charged, and consequently, different surface regions may lead to different charging 

shifts. 

An efficient and nondestructive way to compensate for charging is by increasing the 

films’ conductivity in the presence of simultaneous laser light irradiation [59]. We are using 

the second harmonic (SH) wavelength, at ca. 400 nm, of a Ti:sapphire laser, at 83 MHz 

repetition rate, which is synchronized to the BESSY multi bunch repetition rate, 500 MHz. 

Details as to the laser systems, including aspects of pulse synchronization, are presented 

in Sec. 6. 400 nm wavelength is well within the first optical absorption band of 6T films 

(Figure 2.7), corresponding to exciting electrons across the energy gap (from the Highest 

Occupied Molecular Orbital to the Lowest Unoccupied Molecular Orbital, HOMO-LUMO). 

This fosters the creation of excitons - the charge carriers in organic materials [45] - that move 

throughout the film. Excitons can dissociate at grain boundaries or any defect site in the film, 

or simply from the movement itself (sometimes referred to as a hopping mechanism [60]). The 

charges then diffuse in opposite directions; the electrons, experiencing the surface potential, 

head for the surface, whereas the holes migrate to the substrate. This may, however, not be 

the only mechanism leading to an increased conductivity of the organic film. Since only about 

10 % of the laser light 6 can be absorbed by the film, the remaining light can optically excite 

6 This can be determined according to the Beer-Lambert law: 

I 0 

I = expA (4.2) 

Here A is the absorbance and I 0 and I are the initial and final transmission intensities. Our 6T absorption 

measurements showed that for λ ≈ 400 nm our 6T film had an absorbance of 0.11, thus taking the inverse 

of the above equation the transmitted light (T) can be calculated as follows: T= I 

I 0 

=1/exp 0.11 . This yields 

a transmission of the light to the metal surface of 89.6%, i.e. 10.4% absorbed in the film.


metal electrons, promoting them into the LUMO of the organic film (internal photoemission). 

Figure 4.9 presents two sets of photoemission spectra for 2000 Å 6T film, each of which 

contrasts the effect of laser on (bottom) vs off (top). The pair of spectra on each tier contrasts 

the E vector of the synchrotron light (E), being either perpendicular (blue) or parallel (red) 

to the 6T long molecular axis. All spectra were consecutively measured at grazing incidence 

in normal emission, obtained for 50 eV photon energy. The maximum laser power incident 

on the organic sample was 350 mW which, however, was reduced by optical filters (typically 

to 30-300 mW, depending on single or multi-bunch modes), as higher pulse energies won’t 

improve the spectral quality. The focal spot size of the laser beam at the sample surface 

was about 1mm diameter which is considerably larger than the synchrotron focus (see Sec. 

3.1), thus warranting that only photoelectrons originating from the laser-irradiated sample 

region were detected. The spectra in Figure 4.9 were normalized to the background signal 

near 25 eV; this was observed to be identical and reproducible. Binding energies are with 

respect to the Fermi level, and the main features are labeled just as in Figure 4.8. Upon 

laser irradiation the organic peaks shift back to their true electron binding energies, and 

obviously the overall spectral resolution is greatly improved. The effect is most noticeable 

for feature A where we observe the emergence of two new distinctive peaks, at 2.0 eV (A’) 

and 2.7 eV (A”) binding energy, arising out of its shoulder. These peaks represent the two 

highest occupied molecular orbitals (HOMO and HOMO-1), respectively. Also, the π-band 

region, B, is considerably sharpened for the laser-excited sample as perceived by two defined 

shoulders, B’ at 3.45 eV and B” at 5.4 eV. Feature D seems least affected, although it clearly 

sharpens, and feature C experiences a particular intensity increase near 8.7 eV. 

The pronounced intensity differences (bottom tier of spectra in Figure 4.9) for the two 

geomtries studied is a clear indication of some considerable order of 6T orientation within our 

(thin) films. This anisotropy correlates to the two corrugated (underlying) main azimuths of 

the substrate, [110] and [001], which coincides with the long and short 6T axes, respectively. 

Notice that the charging shift quantitatively applies equally to the two geometries. 

The inset in Figure 4.9 shows the photoemission anisotropy of the clean gold substrate, 

which is provided for comparison. The set of PE spectra were, again, obtained for the 

synchrotron light polarization vector being parallel (top) or perpendicular (bottom) to the


Photoemission Signal [arb. u.] 

160x10 3 

140 

120 

100 

80 

60 

40 

20 

0 

Laser OFF (dark) 

D 

Laser ON 

D 

Mass Thickness ca. 2000 Å 6T 

C 

-16 -14 -12 -10 -8 -6 -4 -2 


C 

E 

E 

B 

[110] 

[110] 

B'' 

100x10 3 

A 

80 

60 

40 

20 

0 

B' 

-8 -4 0 


A'' 

A' 

Fig. 4.9: Photoemission spectra of a ca. 2000 Å 6T film with (bottom) and without (top) the 

addition of 400 nm laser irradiation. PES obtained for 50 eV photon energy measured in normal 

emission. The incoming synchrotron light was at a grazing angle with the polarization vector 

being either perpendicular (blue) or parallel (red) to the long axis of the 6T molecules. Here the 

laser-irradiated spectra show a rigid shift back to the true binding energy values as displayed here 

with respect to Fermi edge. Additionally, an overall resolution improvement is achieved as best 

illustrated through the appearance of the HOMO (2.0 eV) and the HOMO-1 (2.7 eV). 

main azimuths, [110] and [001], respectively. The photon energy was also 50 eV, and the 

spectra were normalized at the background signal near 12 eV. The important point is that 

the anisotropy effect of the Au-5d signal is just opposite to that observed for the organic 

film. The Au signal near 4 eV drastically increases for the ’parallel’ geometry but the signal 

from the organic film near 4 eV drops for the same geometry. 

Synchrotron Flux. The observed charging shift of about 2 eV in Figure 4.9, quantitatively 

reflects the particular synchrotron flux used to record these spectra; the higher 

the flux the more charges may be generated, and hence larger shifts would be measured. 

Figure 4.10 illustrates the range of synchrotron-induced spectral shifts that we encountered 

as a function of the synchrotron photon flux (see labels). The valence band PE spectra of 

ca. 2000 Å 6T/Au(110) were subsequently recorded in normal emission, using 50 eV photon 

energy, without laser irradiation. Synchrotron flux variation was accomplished by de-tuning 

the undulator gap. The total change was by a factor of 100 between the top and bottom 

spectra. The bottom spectrum was obtained for the highest flux, corresponding to the high-


Photoemission signal [arb. units] 

350x10 3 

300 

250 

200 

150 

100 

50 

0 

140x10 3 

-20 -15 -10 -5 0 


120 

100 

80 

60 

40 

20 

0 

-15 -10 -5 0 


0.50 nA 

0.74 nA 

1.20 nA 

2.19 nA 

2.70 nA 

5.10 nA 

8.90 nA 

13.8 nA 

19.2 nA 

24.0 nA 

54.0 nA 

Fig. 4.10: PES spectra of 2000 Å film of 6T/Au(110), obtained for 50 eV photon energy, recorded 

as a function of synchrotron photon flux to illustrate the large range of induced spectral shifts. 

Comparing the maximal and minimal (top) photon fluxes the resulting charge shift varies by a 

factor of ten. This latter figure indicates that charging is always present. The provided inset 

emphasizes the smearing of the spectra in the maximal and minimal flux comparison. 

est ring current available (ca. 250 mA). The actual currents indicated in the figure refer to 

the currents measured by a gold mesh that could be driven in front of the sample. 

The observed spectral shifts range from 1.75 to 5.0 eV for the lowest and highest synchrotron 

flux, respectively. I. e., even for the lowest flux studied here a considerable charging 

shift exists. Notice that the peak positions of the non-shifted features were determined from 

thinner films (see Figure 4.8). 

The inset in Figure 4.10 serves as a direct illustration of 

the maximum spectral smearing. It displays the spectra obtained for the lowest vs highest 

synchrotron intensity on top of each other (and suitably shifted). Typically, for our angleresolved 

measurements conditions were used corresponding to about 1 nA (third spectrum 

from top). 

We quickly mention the possibility of radiation damage induced by synchrotron light as 

this is a common concern in studying organic molecules with intense EUV/XUV light [59], [61]. 

The effect has been generally ascribed to the interaction of secondary electrons with the organic 

film. We observed only minor radiation damage over extended periods of synchrotron 

light exposure times, on a scale much greater than the time needed to record our spectra.



40x10 3 

30 

20 

10 

0 

38 mW 

120 mW 

345 mW 

Ti:Sa, λ = 410 nm 

-14 -12 -10 -8 -6 -4 -2 


Fig. 4.11: PE spectra obtained for a maximal synchrotron ring current (corresponding to ca. 50 

nA mesh current) at 50 eV photon energy as a function of laser power. The numbers show the 

power of the laser at the sample, being adjusted by various filters before entering the chamber. 

Even with the weakest laser pulses a full compensation of all charging is achieved, i.e. all peaks 

(specifically the localized π-band at 4.1 eV) remain at a constant binding energy. 

In spite of that we have frequently moved the sample in order to expose a new surface spot. 

Effect of laser fluence. Notice that for the experimental conditions in Figure 4.9 the 

laser pulse energy was sufficient to fully compensate for the charging shift. Figure 4.11 

explores the charge compensation, for highest synchrotron flux (comparable to lowest trace 

in Figure 4.10), as a function of the laser power, which has been adjusted using optical 

filters 7 . 

Obviously, the laser compensates for all levels of charging obtained (see dashed 

line), even at 10 % of maximum power. True, non-shifted peak positions are obtained for all 

laser fluences shown. 

7 For our set-up, the laser transmission is logarithmic depending on the inserted filter according to 

T=10 (−Filter) . So, for our 380 mW Ti:Sa laser passing through a 0.5 filter is as follows: T· (380mW)=10 (−0.5) . 

This yields a laser power that reaches the sample of 120 mW.

4.3 Angle-Resolved Photoemission from 6T/Au(110): Molecular 

Orientation 56 

4.3 Angle-Resolved Photoemission from 6T/Au(110): 

Molecular Orientation 

The success of obtaining clear HOMO, HOMO-1, and other spectral features in our PE 

spectra of sexithiophene multilayers on Au(110), in the presence of laser irradiation, gives 

us the sensitivity necessary to observe distinct feature evolution (intensity variations) as a 

function of the experimental geometry. 

The main experimental parameters are direction 

of the light polarization vector and angle of electron detection both with respect to the 

azimuthal orientation of the sample and to the surface normal. Then, by applying group 

theory and symmetry-derived selection rules to the PE data, we can infer details on the 

orientation of the 6T molecules within thin films. 

4.3.1 Measured Photoemission Anisotropy 

Angle-resolved photoemission was typically measured for grazing photon incidence (Ψ i = 

7 ◦ fixed) at varied emission angle (0 < θ e < 20). Figure 4.12 is a sketch of the principal 

experimental geometries, E n D n , used for angle-resolved PE. Here E and D refer to SR 

polarization vector and detection. The subscript n = x, y, z, for E, stands for the direction 

of the polarization vector of the incident SR light; for D, n refers to the photoelectron 

detection plane, θ e . The coordinate system is defined with x as the direction along the long 

6T molecular axis, y as across the molecular axis, and z as the direction perpendicular to 

the crystal surface. Notice that at normal emission (θ e = 0) the photoelectrons are always 

detected along the z-axis; then the detection plane refers to angle measurements (θ e > 0) in 

the plane that follows along or cuts across the 6T molecular axis. For example, when the 

polarization of the light is in the x-direction and the emitted electrons are detected in the x-z 

plane, the corresponding geometry would be E x D x . Geometries with the light polarization 

vector in-plane are referred to as even, while geometries with a perpendicular component to 

the crystal surface are denoted odd (see fig. 4.12 labels). In Fig. 4.12 the 6T molecules on 

Au(110) are depicted as lying flat. The long axis of the molecule is along the direction of 

the missing row of Au atoms, [110]. 

Displayed in Figure 4.13 are the resulting photoemission spectra for various E n D n combinations. 

All spectra were obtained for 50 eV photon energy during single bunch using

[110] 



ExDx 

Z 

even 

EyDy 

Z 

even 

θe 

θe 

E 

[001] 

Y 

[110] 

X 

E 

[110] 

X 

[001] 

Y 

X 

odd 

Y 

odd 

θe 

[110] 

θe 

[001] 

Z 

Z 

EzDx 

[001] 

E 

Ψ i 

Y 

Ψ i Ψ i 

Ψ i 

EzDy 

E 

X 

Fig. 4.12: A sketch of the four main experimental geometries used in gathering the angle-resolved 

PE spectra. The simple scheme shows the direction of the incoming polarization vector ( −→ E ) with 

respect to the long axis of the lying 6T molecules. This corresponds to the parallel [110] or 

perpendicular [001] main axes of the substrate. The z-direction is always in the plane of detection 

with an angle for the ejected electron represented by θ e . 

simultaneous 2.2 eV laser irradiation (Nd:YVO 4 ). The upper and lower frames represent 

the even (A) and odd (B) geometries, respectively. The top two spectra in (A), obtained for 

normal emission, contrast in the azimuth of the sample, positioning the light polarization 

vector either along (E x D x ) or across (E y D y ) the long axis of the molecules. The bottom tier 

of even-geometry spectra (lower solid lines) in (A) was measured 20 ◦ off normal emission, 

within the x-z and y-z plane, respectively. The dotted spectrum is a reproduced E x D x at 

normal emission to guide the reader in the comparison. The pairs of odd-geometry spectra, 

in (B), again illustrate the differences between photoelectron detection at normal and 20 ◦ 

off emission. Here the polarization of the light was held constant in the z-direction, and the 

detection was either in the y-x or x-y plane. The dotted spectrum, again a copy of E x D x in 

normal emission, serves to exemplify the normalization between the two groups of spectra.



Peak E x D x (even) E y D y (even) E z D x (odd) E z D y (odd) 

A 1.0 (1.0) 0.5 (0.5) 1.0 (2.0) 1.0 (1.0) 

B 1.0 (1.0) 1.0 (1.0) 2.5 (1.0) 2.5 (1.5) 

C 1.0 (1.0) 1.3 (1.25) 1.5 (1.25) 1.5 (1.2) 

D 1.0 (1.0) 1.0 (1.0) 1.0 (1.2) 1.0 (1.0) 

Tab. 4.2: Quantified summary of the relative intensity variations of the geometry-dependent 

photoemission signal at normal and at 20 ◦ off normal emission (in parenthesis). All numbers are 

with respect to the reference spectrum E x D x , within which all spectral features were assigned to 

1.0. 

This enables quantitative comparisons between the in(even)- and out(odd)- of plane geometries. 

Normalizing the background signals of E x D x with E z D x , and then using the latter as 

the sub-reference spectrum, as portrayed by the traced E z D x (dashed) spectrum, we could 

relate all peak intensity variations to one single reference. 

Typically the signal in the π region of HOMO, HOMO-1, and HOMO-2, at approximately 

1.85 eV, 2.55 eV, and 3.3 eV, respectively, is greatly enhanced in the x-detection plane, 

regardless of the detection angle. A reversal effect, i.e. a stronger D y signal, is seen in the 

even spectra between the σ-regime of 5.7 eV and 8.0 eV binding energies, and in the odd 

spectra at the shoulder at 5.3 eV. The deeper σ-band, 11.5 eV binding energy, becomes nearly 

identical for all geometric combinations. Also note the odd spectra of E z D y and E z D x ; this 

particular geometry innately has the same excitation light vector and the identical electron 

emission direction. Thereby the rotation of the samples’ main azimuths should have no effect 

for an electron being ejected from the surface. Hence, one would expect the mirrored image 

of these spectra that we do, in fact, observe. The peak at 5.3 eV is the only exception, 

which is attributed to the fact that for any E z D n geometry the light polarization vector is 

not fully perpendicular to the crystal surface (otherwise the SR light wouldn’t strike the 

crystal). However, the fact that all other features match assures that this effect is negligible, 

especially in the crucial upper energy states. 

A quantitative summary of the aforementioned geometry-dependent relative intensity 

variations obtained for 6T/Au(110) is presented in Table 4.2. Here all numbers are with 

reference to E x D x , within which all spectral features were assigned to 1.0. In parenthesis 

are the relative intensities for the same principle geometry, however measured at 20 ◦ off 

normal emission. Here it is re-iterated that in the even geometry there is a decrease in



(A) 

even 

ExDx norm emiss (NE) 

EyDy norm emiss (NE) 

10x10 3 8 


6 

4 

2 

0 

ExDx 20° off NE 

EyDy 20° off NE 

ExDx NE 

HOMO-1 

HOMO 

-14 -12 -10 -8 -6 -4 -2 0 


15x10 3 

(B) 

odd 

EzDx NE 

EzDy NE 

ExDx NE 

(in-plane) 


10 

5 

0 

0 

EzDx 20° off NE 

EzDy 20° off NE 

EzDx NE 

HOMO-1 

HOMO 

-14 -12 -10 -8 -6 -4 -2 0 


Fig. 4.13: Photoemission spectra from ca. 2000 Å 6T/Au(110) film obtained in several experimental 

geometries for 50 eV photon energy with an additional 2.2 eV laser irradiation. The top 

and bottom graphs contrast in even and odd geometry, respectively (see Fig. 4.12). Each set of 

spectra (solid lines) illustrates the intensity difference depending upon the direction of the light 

polarization vector, and each tier of spectra compares the detection at normal and 20 ◦ off normal 

electron emission. The dotted and dashed spectra are reproductions of the reference spectra used 

for normalization purposes, and are shown here to help guide the reader in spectra comparison.



average signal intensity of the HOMO and HOMO-1 from 1.0 to 0.5, and the increase of the 

deeper lying orbitals (σ-system) from 1.0 to 1.3 when going from E x D x (reference) to E y D y , 

respectively. In the odd geometry there is a x2.5 increase of the large π-feature (4.1 eV) in 

normal emission in both detection planes, and again a doubling of the HOMO and HOMO-1 

intensity in just the x-detection plane. These particularly pronounced differences for the two 

principal azimuths, [001] vs [110] directions, need to be examined in detail. Thus we first 

need to quantitatively describe the system through valid simulations to compare later with 

our physical experimental data. 

4.3.2 Calculated Molecular Orbitals: Binding Energies, Characters, 

and Symmetries 

There are two main steps that need to be accomplished in order to perform accurate energy 

calculations, which can then be directly applied to the experimental evidence for analyzation. 

First the sexithiophene must be constructed in a likeness of a realistic 6T molecule. Then 

the resulting molecule must be geometrically optimized in accordance with certain symmetry 

assumptions. 

Completing this procedure yields, in addition to the optimized molecular 

structure, the total energy and likewise the energies of the wave functions describing the 

molecule. It is this latter calculation that predicts the ionization potentials, which through 

equation 2.3 infers the electronic structure. 

A critical variable in the beginning of the simulation is the aforementioned molecular 

symmetry. The symmetry elements of the molecule (Sec. 2.4) can be controlled within 

the optimization process of the bond distances, bond angles, and dihedral angles. The two 

extreme conformers created by adjusting only the inter-ring torsion leads to molecular symmetries, 

C 2h and C 2v , corresponding to trans and cis, respectively (see Sec. 2.3). A broken 

or reduced symmetry, such as C 2 or C s , can be also evaluated as intermediate-conformer 

possibilities. In the latter case the 6T molecule is no longer completely planar but rather 

adjacent rings are tilted with respect to each other. Thus, by simply adjusting the molecules’ 

dihedral (torsion) angles, sexithiophene molecules were constructed with the instilled symmetry 

elements innate to C 2h , C 2v , and C 2 point groups. These symmetries for n-thiophenes 

have already been reported in the literature and were shown to represent energetically stable



molecules: C 2h in bulk studies [43] and C 2v symmetry for odd n-thiophenes in bulk [29], 4T 

in solution [62], and 2T for sub-monolayer [6] and other [63]. Therefore, the likely presence of 

a number of similar 6T conformers led to a scrutiny of these possible symmetries. Moreover, 

although the final goal is the calculation of MO energy values, assuming a given symmetry, to 

assure accuracy of the electronic structure one has to also optimize the molecular geometry 

thereby providing a more realistic scenario. Therefore, in addition to setting the dihedral 

angles, initial conditions for the other two parameters, bond distances and bond angles, were 

assumed as standard values. This process was first applied to a trans-bithiophene molecule 

(2T) because the molecule itself is smaller but still possesses very similar characteristics to 

the studied 6T (see Sec. 2.3). The advantage of this becomes clear as we investigate the 

extrapolation of this molecule into the larger 6T. 

Using the described basis set and Hartree-Fock method (see Sec. 2.5), a trans-bithiophene 

molecule was constructed and optimized. For our purposes in the construction of this transconformer, 

along with complementary standard values for the bond angles and distances, 

symmetry demanded a 180 ◦ dihedral angle (C 2h ) as the initial parameter. The resulting 

optimized gas-phase bithiophene (2T) molecule is presented in Figure 4.14. Here are the 

optimized distances (Å) and bond angles (degrees) detailed; for example the bond distance 

between the two carbon atoms at the bridge position connecting the rings (C α -C α ) is 1.46 

Å and the angle for the sulfur within the ring is 91.6 ◦ . These values are similar to those 

reported by Fujimoto et. al. [29]. 

The trans conformer in Fig 4.14 depicts only one of n-thiophene’s possible symmetries. 

To adjust to a desired symmetry, for our purposes between C 2h and C 2v , we simply rotate 

the central thiophene ring about its axis, or in practice varying the dihedral angle between 

the two rings within bithiophene. 

Again, the Hartree-Fock calculations deliver not just 

geometrical optimizations but also the energy of the individual orbitals and the sum of these 

comprised orbitals as the molecule’s total energy, or mathematically 

∑ 

n i E i = E total , n = 0, 1, 2, (4.3) 

i 

where n is the electron occupation number per i orbital. So, in examining E total , a potential 

energy surface for bithiophene conformers was calculated, as dependent upon inter-ring torsions. 

The result is presented in Figure 4.15, calculated using the same procedure, as will be



Fig. 4.14: Using the Hartree-fock method with a 6-31G(d) basis set, a bithiophene molecule was 

geometrically optimized. The original construction fixed a C 2h symmetry (trans) where the torsion 

angle between the two rings is 180 ◦ . Gas-phase optimized bond lengths and angles are shown in 

the top and bottom sketches, respectively. 

the case for all other constructed n-thiophenes throughout this work - that is applying the 

Hartree-Fock model with a 6-31G(d,p) basis set. It should be mentioned that this HF model 

with the appropriate basis set generally yields particularly good descriptions of molecular 

conformer energies [49]. The inset pictures show the corresponding conformer at the energy 

minima and maxima. The difference between the symmetries in question, C 2h (trans) vs C 2v 

(cis), is quite small, ca. 0.03 eV, with only a 0.08 eV potential barrier between the trans and 

cis respective conformers which have a reduced C 2 symmetry. In fact, at room temperature 

one is tempted to propose the presence of a 1:8 ratio of strict C 2v :C 2h bithiophene-conformers 

and likewise as high as a 1:3 ratio for their C 2 -symmetric counterparts. Similar calculations 

have been performed, arriving at very similar conclusions [6], and also by applying density 

function calculations by Telesca et. al. [31]. Thus the existence of various conformers in our 

film seems reasonable. 

Especially note that our measurements are quite surface-sensitive. Hence our data will 

contain considerable interfacial contributions. This interface between vacuum and organic 

film is likely to create an environment favoring a particular conformer. As shown in the sketch



0.12 

0.10 

0.08 

Relative Energy [eV] 

0.06 

0.04 

0.02 

0.00 

-0.02 

HF/6-31G(d) 

190 170 150 130 110 90 70 50 30 10 

Torsion angle [degrees] 

Fig. 4.15: Potential energy curve of bithiophene according to torsion angle, calculated using the 

basis HF/6-31G(d). Inset cartoons show the corresponding conformer by the maxima and minima 

in the graph. The far left and far right sides of the torsion axis represent C 2h and C 2v symmetries. 

for 2T in Figure 4.15, for example, a C 2v symmetry would provide an atmosphere that leaves 

only hydrogen bonds dangling at the surface, burying all the sulfur atoms. It could be then 

plausible that in such a case the energy relaxation of the sulfur atoms would be large enough 

to overcome the weak potential barrier separating the two highly symmetric conformers. In 

summary, we have to further investigate the system for sexithiophene considering various 

symmetric conformers, especially since 6T can exist as various conformers each possessing 

either a C 2v or C 2h symmetry. 

It is interesting now to extrapolate our constructed 2T into a (four-ring) larger sexithiophene. 

Notice that within the π-system, both localized and delocalized π-orbital characters, 

will split into two energy levels (again anti-bonding and bonding) upon the addition of a 

thiophene ring [29], [31]. In practice, by expanding our already constructed bithiophene by 

2n-thiophene rings, up to our studied sexithiophene, an equal amount of 2n π-orbitals would 

then be expected. To accomplish this, a quarterthiophene (4T) and a sexithiophene (6T) 

molecule were built, two rings at-a-time, to ensure the symmetry of the entire molecule 8 . 

8 It should be noted that similar studies performed by Telesca et. al [31] claim that the systematic addition



Because the 6T molecule is the molecule in question for this work, only the optimized 6T 

is shown in Figure 4.16. Since the molecule was built after the trans-2T model, the trans 

(C 2h symmetry) 6T conformer is shown. Figure 4.16 displays the optimized bond lengths 

(Å) and angles ( ◦ ) for the final model sexithiophene in the same manner as Figure 4.14. Due 

to the molecule’s C 2 symmetry only half of the rings are shown, in which the far left and 

far right rings correspond to the end and middle rings, respectively. The largest differences 

in bond lengths and angles relative to each other are found in the terminal rings; the inner 

four rings have almost the identical structure. For example, the bond distance between the 

two carbons which are within the same ring (C β -C β ) for the inner-rings is 1.460 Å, and the 

bond angle for the inner-ring sulfur atoms is 92.0 ◦ . The two outer-rings show an increase in 

the same C β -C β bond distance of 0.005 Å, and likewise an increase in the sulfur bond angle 

of 0.4 ◦ . The outer-ring dimensions do not greatly differ from their inner-ring counterparts; 

notice however, that the dimensions of the two outer-most rings 9 are analogous to those of 

the modeled 2T (see Figure 4.14). 

In order to highlight the energetic splitting of the bithiophene molecular orbitals as a 

function of 2n-thiophene rings, Figure 4.17 shows the calculated (HF/6-31G(d,p)) MO energy 

levels for the π-system of bithiophene (2T), quaterthiophene (4T), and sexithiophene (6T). 

The electron binding energies are with respect to the Fermi energy 10 , E F , which is represented 

by the light dashed line at 0 eV. Here we confirm the expected number of orbitals in the 

π-system, that is to say 4, 8, and 12 π-bonds for 2T, 4T, and 6T, respectively. The tight 

band of eigenstates (near 4-5 eV) corresponds to the localized (n) electron states, and the 

out-lying eigenstates (above and below) are the delocalized (π) states (see labels). Note that 

particularly the localized (n) states do not split linearly, i.e. even after adding thiophene 

units the new n-eigenstates remain centered around 4.1 eV. This change in the BE (∆E) 

for the entire range of localized (n) states in 2T is only ca. 0.23 eV (∆E loc(2T) ). Whereas 

when comparing the ∆E delocalized states - that is comparing the extreme (highest and 

lowest energy) π-levels roughly speaking - we observe a splitting ten times larger than the 

of only one identical monomer unit (for oligomers of thiophenes) yields negligible symmetry effects. 

9 Furthermore, similar results have been previously reported [3], [29] as evidence that end effects are 

localized on the terminal rings. 

10 This is assigned to 5.37 eV, the work function of Au(110) [64].



... 

... 

Fig. 4.16: Schematic of the optimized geometry calculated (HF/6-31G(d)) for the 2T-extrapolated 

trans-6T conformer. Only three rings are shown, due to the C 2 symmetry of the molecules, with 

the left- and right-most rings being the end and middle rings in the chain, respectively. Bond 

lengths (top) and bond angles (bottom) are displayed. 

localized states, ∆E del(2T) ≈ 2.6 eV. This ratio holds for 6T as well; this same ∆E value for 

the delocalized (π) states for 6T doubles, ∆E del(6T) ≈ 4.7 eV and is ten times greater than 

the localized splitting, ca. 0.44 eV (∆E loc(6T) ). 

Both the π-system’s localized and delocalized states are pictorially represented in the 

cartoon on the right side of Figure 4.17. Only the p z orbitals are drawn (for three adjacent 

rings of the molecule, n, n+1, n+2), since these are the key wavefunctions responsible for 

the localization of orbitals (”cross-talk” between rings) 11 . Here the delocalized orbitals can 

have either a bonding (symmetric) or anti-bonding (anti-symmetric) character (see Sec. 2.5). 

The bonding MO (bottom) is the energetically most stable with the highest probability of a 

constructive overlap. This allows this one-electron wavefunction to be extended across the 

α-carbon atoms and the other (both α- and β-) carbon p z orbitals on the n+1 ring [65]. 

On the other extreme are the anti-bonding MO’s (top), where the probability of finding an 

electron on the same side of its orbital’s node, but in a direct neighboring wavefunction, is 

11 Combinations of the π x - and π x -wavefunctions will result in σ-bonds (as opposed to the π-system investigated 

here).



1 

E f 

Pz orbital 

n ring n+1 n+2 ring 

-1 

Electron Binding Energy [eV] 

-2 

-3 

-4 

-5 

∆ 

∆ 

E del(2T) 

E loc(2T) 

Anti-bonding 

( π) 

∆E del(6T) 

Non-bonding 

(n) 

∆E loc(6T) 

-6 

Bonding 

( ) 

π 

-7 

0 2 4 

N (number of rings) 

6 

Fig. 4.17: The left-hand side of the figure contains a graph comparing calculated 2T, 4T, and 

6T π-orbital energies and the splitting within these states as a function of the respective n-rings. 

Here it is easy to identify the small splitting (∆E 6T ≈ 0.44 eV) of the localized π-states and the 

increasingly larger spread (∆E 6T ≈ 4.7 eV) of the delocalized π-states. This is illustrated by the 

cartoon of p z orbitals in the right-hand side of the figure. This spatial combination is the crucial 

aspect in determining anti-, non- or bonding characters. The anti- and bonding characters are 

the extreme splitting cases. Whereas the appropriately named non-bonding peaks remain separate 

from interactions with neighboring rings and hence are unaffected by n-ring size, yielding a smaller 

splitting. 

very low, even when the neighboring atom is identical. These wavefunctions, in turn, overlap 

destructively; thus destabilizing the orbital by a particular energy, driving the energy level 

higher. Just to remind the reader, regardless of bonding or anti-bonding, when molecular 

orbitals are between (only) α-carbon atoms, the orbitals are said to have an aromatic character 

(Sec. 2.3). Also, notice the number of bonding and anti-bonding π-states; there are 

three of the latter and only two of the former π-states as depicted in Figure 4.17. There 

must exist an equal number of these two types of π-states, which implies a ”missing” third 

bonding eigenstate is among the tight band of n-states. 

Furthermore, returning to the middle of the cartoon (Fig. 4.17), clearly sketched are the 

non-bonding (n) or localized orbitals (the tight band of energy levels), which, as their name 

implies, do not bond with their neighbors. In this case the atoms taking part in the molecular 

orbital are no longer only carbons. The larger sulfur atom is contributing its p z orbital, which 

consequently interrupts the continuity along the otherwise homogeneous p z orbital-system



supported by the carbon atoms. This anchors the electrons at certain positions belonging to 

individual rings within the molecule. This particular property is known as quinoid orbital 

character (see Sec. 2.3). 

To visually illustrate the details of aromatic and quinoid characters, the simple cartoon 

in Figure 4.17 is somewhat insufficient. For the optimized 6T molecule (Figure 4.16) we generated 

the molecules’ exact orbital pictures, a selection of which is displayed in Figure 4.18. 

It is then obvious to see the described aromatic and quinoid orbital characters simply by 

looking for which atoms are participating in forming the molecular orbitals. The figure separates 

the orbitals according to character, with aromatic and quinoid orbital character on 

the left and right hand sides of the binding energy scale, respectively. Note that the energy 

scale is not drawn linearly. Clearly the highest three occupied orbitals exhibit the aromatic 

character, specifically where only the carbon atoms interact in fostering the orbitals. Again 

to remind the reader, Figure 4.17 clearly presents that for our system of six conjugated thiophenes, 

we should expect twelve π-energy states, which break down into three bonding, six 

non-bonding, and three anti-bonding orbitals. This latter character describes the MO’s at 

the top of the energy scale. Conversely, the two deepest lying π-states, at 5.35 and 6.18 eV, 

depict the pronounced overlap of orbitals between rings along the chain, that is essentially 

the strong attractive nature of the bonding character. The remaining delocalized orbital 

(our thus far ”missing” third bonding MO) is located among the localized MO’s centered 

around 4.1 eV. This is in Figure 4.17 completely indiscernible; however, creating such orbital 

pictures (Figure 4.18) allows us to conclude the location of this last delocalized (bonding) 

π-level, simply by looking for the aromatic character. So, upon examining the MO’s on the 

right side of Figure 4.18, it is easy to identify the six non-bonding MO’s, in so far that they 

involve the sulfur atoms or preferably, are quinoid in character. It is also straightforward to 

pinpoint the one aromatic MO at 4.34 eV and assign it to the third bonding 6T MO. The 

last MO pictured in the lower right corner box of Figure 4.18 is exemplary of a σ-MO where 

the orbital is symmetric with respect to the molecular plane. 

It is crucial at this point to recall that all the above figures and calculations refer to the 

optimized trans 6T molecule, which was extrapolated from the trans bithiophene. Again, 

the HF model must be separately applied to molecules with dissimilar symmetries. Remem-



Aromatic 

BE(eV) 

1.5 

2.25 

Quinoid 

*Energy values not to scale 

3.27 

4.04 

4.07 

4.16 

4.34 

4.28 

4.40 

4.48 

5.35 

6.18 

7.19 eV 

Fig. 4.18: A schematic of a reconstruction of actual molecular orbitals found in the π-system 

of a sexithiophene molecule with C 2h symmetry (symmetry arbitrarily chosen). The MO’s are 

shown as straddling the energy scale and are separated by character with the localized (right-side) 

and delocalized (left-side) characters. Quinoid and aromatic MO character can be easily identified 

simply by noting if the sulfur atom does or does not take part in the molecular bonding. The boxed 

picture in the lower right-hand side is an example of a σ-MO, which is symmetric with respect to 

the molecular plane. Binding energies are given with respect to the Fermi edge, notice that the 

energy scale is not linear. 

bering Figure 4.15, different 6T conformers may be expected on the surface of our film. 

Thus new molecular conformers were (in the same manner) geometrically optimized and the 

binding energies for the individual orbitals were calculated. By only adjusting the dihedral 

angle (torsion) of this optimized trans-6T (C 2h ), we constructed two conformers with 

C 2v symmetry, the energetically unfavorable all-cis configuration and the more probable C 2v 

configuration where only the middle rings are (structurally) exchanged. Additionally investigated 

were the two reduced states of the respective high-symmetries, C 2 and C s . A 

comparison of all the calculated values is summarized in Table 4.3. The binding energy (eV) 

values are presented with respect to the Fermi energy 12 ; the character of the orbitals is also 

shown in parenthesis. 

It should also be said that a slight red-shift of binding energies is expected for HF cal- 

12 Taking into account the work function of Au(110), 5.37 eV.



Orbital C 2 (eV) C 2h (eV) all-C 2v (eV) C 2v (eV) C s (eV) 

HOMO (π) -1.75 (B) -1.54 (B g ) -1.38 (A 2 ) -1.50 (A 2 ) -1.51 (A”) 

H-1 (π) -2.47 (A) -2.31 (A u ) -2.22 (B 1 ) -2.31 (B 1 ) -2.30 (A”) 

H-2 (π) -3.39 (B) -3.33 (B g ) -3.27 (A 2 ) -3.32 (A 2 ) -3.33 (A”) 

H-3 (π) -4.15 (A) -4.10 (A u ) -4.00 (A 2 ) -4.08 (A 2 ) -4.07 (A”) 

H-4 (π) -4.16 (B) -4.12 (B g ) -4.05 (B 1 ) -4.11 (B 1 ) -4.11 (A”) 

H-5 (π) -4.23 (A) -4.22 (A u ) -4.16 (A 2 ) -4.20 (A 2 ) -4.21 (A”) 

H-6 (π) -4.28 (B) -4.33 (B g ) -4.22 (B 1 ) -4.33 (B 1 ) -4.32 (A”) 

H-7 (π) -4.31 (A) -4.40 (A u ) -4.46 (B 1 ) -4.39 (B 1 ) -4.40 (A”) 

H-8 (π) -4.37 (A) -4.45 (A u ) -4.47 (A 2 ) -4.43 (A 2 ) -4.45 (A”) 

H-9 (π) -4.39 (B) -4.53 (B g ) -4.58 (B 1 ) -4.58 (B 1 ) -4.56 (A”) 

H-10 (π) -5.16 (B) -5.41 (B g ) -5.38 (A 2 ) -5.41 (A 2 ) -5.41 (A”) 

H-11 (π) -5.74 (A) -6.24 (A u ) -6.20 (B 1 ) -6.23 (B 1 ) -6.23 (A”) 

H-12 (σ) -7.33 (A) -7.24 (A g ) -6.91 (B 2 ) -7.19 (B 2 ) -7.17 (A’) 

H-13 (σ) -7.37 (B) -7.30 (B u ) -7.08 (A 1 ) -7.30 (A 1 ) -7.29 (A’) 

H-14 (σ) -7.46 (A) -7.42 (A g ) -7.33 (B 2 ) -7.39 (B 2 ) -7.47 (A’) 

H-15 (σ) -7.59 (B) -7.57 (B u ) -7.58 (A 1 ) -7.65 (A 1 ) -7.61 (A’) 

H-16 (σ) -7.79 (A) -7.77 (A g ) -7.77 (B 2 ) -7.85 (B 2 ) -7.86 (A’) 

H-17 (σ) -8.01 (B) -8.08 (B u ) -7.92 (A 1 ) -8.11 (A 1 ) -8.11 (A’) 

Tab. 4.3: Summary of the calculated eigenvalues for five symmetry possibilities: C 2 , C 2h , all-C 2v , 

C 2v , and C s . Calculations were realized through a 6-31G(d) basis applied in the Hartree-Fock 

method. All π-orbitals and a sample of some σ-orbitals are shown, as well as the corresponding 

molecular orbital character (as marked in parenthesis). All numbers refer to binding energies after 

conversion into eV and after taking the Au(110) work function into consideration. Note that the C s 

conformer is exemplified through a 121221 configuration, assuming that the sulfur atom is planar 

but pointing in direction 1 or 2 (its opposite). 

culations [49]. The Hartree-Fock method is considered a closed-shell method, in which the 

motions of individual electrons are treated as independent from all the other electrons. This 

means that the coupling of individual electrons, i.e. electron correlation, is neglected in the 

HF model. Coupled electrons will feel less of an electron-electron repulsion force and, consequently, 

lower the orbital and total molecular energy values away from the overestimated 

values calculated with the HF model. 

The largest difference in binding energies is a ca. 0.35 eV red-shift between the all-cis 

conformer and the broken C 2 . This can be explained within our simulations by the varying 

overlap matrix; the binding energy values tend to shift to higher energies with symmetry 

complexity. All the other conformers, though, possess very similar MO energies. In fact the 

C 2v and C s conformers yield almost identical binding energies to those predicted for the bulk



C 2h structure. 

In summary, we have performed a series of energy calculations on various 6T conformers. 

We have provided the reader with a thorough account of the step-by-step processes involved 

in the calculations. This includes the theory of the applied Hartree-Fock method and its 

advantages, for our chosen basis set, and how this model fairly accurately approximates 

three aspects within our system: (1) The optimization of the molecules’ geometry that in 

turn leads to (2) the prediction of the energy for the entire molecule, which is simply the 

sum over (3) all the individual molecular orbitals. 

The model was applied separately to 

five different conformers of sexithiophene, representing various symmetry point groups. We 

closely investigated the orbital characters - localized, delocalized, bonding, non-bonding, and 

anti-bonding - of the π-system of 6T. These abstract calculations will now be applied to the 

angle-resolved PES experimental results. 

4.3.3 Photoemission Data and Computation Comparison 

On the basis of the above binding energy calculations, for various conformers, we can now 

assign the photoemission spectral features and interpret the observed intensity variations. 

All of the calculated binding energy values in Table 4.3 are displayed together with a pair 

of normal emission reference spectra, E x D x and E y D y , in Figure 4.19. We specifically find 

a good qualitative agreement in the π-system region (as marked), and easily recognize the 

delocalized (first three MO’s) and localized (the MO bunch) energy levels. The σ-states 

set in at ca. 

7 eV, although we observe an earlier signal arising at about 6.6 eV in our 

spectra. Labeled in Figure 4.19 are the molecular orbital characters for the π-region, HOMO, 

HOMO-1, and HOMO-2, and for the σ-region, HOMO-12 and HOMO-15 13 . This highlights 

the difference in MO character between neighboring MO’s and between the two regions; e.g. 

the HOMO and HOMO-1 are B and A, A 2 and B 1 , and B g and A u for C 2 , C 2h , and C 2v 

symmetries, respectively. The same two orbital characters continue to alternate for each 

successive eigenstate within the π-region. For C s , however, the only remaining symmetry 

lies in the molecular plane. Thus all the π-eigenstates assume the same orbital character; 

this π-character is still dissimilar with respect to the σ-orbitals. 

13 All orbital characters are summarized in Table 4.3.



σ 

π 

E x 

D x 

4000 

HOMO-15 

HOMO-12 

HOMO-3 

HOMO-1 

HOMO 

E y 

D y 

3000 

Photemission Signal [arb. units] 

2000 

1000 

0 

A B 

B A B 

B u 

C 2 

A g 

A 1 B 2 

A 2 

B 2 

Bg Au Bg 

B 1 

A 2 B 1 A 2 

A 1 

A 2 

C 2h 

all- 

C 

2v 

-1000 

C 2v 

A' A' 

A" 

A" 

A" 

C s 

-8 -6 -4 -2 0 


Fig. 4.19: Compilation of electron BE of 6T calculated for different symmetries (as labeled). 

These are the symmetries and energies given in Table 4.3. Some of the respective characters are 

marked. Also labeled are the corresponding features, e.g. HOMO and HOMO-1, in the measured 

PE spectra, obtained for 2000 Å 6T/Au(110) in E x D x and E y D y using 50 eV photon energy.



Obviously the accurate peak assignment within the σ-region in the PE spectra is quite 

difficult, mostly due to the broad signal arising from many energy levels, many of which 

partly overlap. Hence, the π-system with its well-sharpened and somewhat independently 

resolved peaks is better suited for a detailed analysis. The calculated binding energies for 

C 2h , C 2v , and C s are all suggested as the good theoretical ”fits”; although the remarkable 

similarity between these symmetries renders them relatively indistinguishable, with the energy 

difference being negligible as compared to the width of the PE features. However, by 

investigating the photoemission signal variations observed for different experimental geometries, 

we can infer details as to which symmetry is favorable. The electron emission direction, 

which is specific for a given orbital character, which in turn is dictated by symmetry selection 

rules, is the key in determining allowed vs forbidden transitions for photoelectron detection. 

To apply the symmetry selection rules, we examine the transition matrix element. 

For our purposes, simply a zero or non-zero matrix element, 〈ψ f |µ|ψ i 〉, for the direct product 

of the initial wave function with the dipole of the light polarization (µ) was of the primary 

concern, and used for investigations. 14 

The resulting wavefunction, ψ f , reveals the detection 

possibilities of an electron ejected from the initial orbital. Mathematically this simply 

corresponds to multiplying each orbital character with the representative character of the 

polarized light, both described by respective symmetry terms (see Sec. 2.4). This process is 

illustrated in Figure 4.20 where ψ i is chosen to have A 2 orbital character (C 2v point group). 

At the bottom of the figure the symmetry element for A 2 character is shown (see Table 2.5). 

The main part of the figure sketches the respective electron density probabilities 15 . Both the 

anti-symmetric vertical mirror (-1) and the symmetric C 2 180 ◦ -rotation (1) are satisfied by 

this drawn probability configuration (very left). Note that the C 2 rotation axis is defined in 

the tables as the z-axis of the system. In the figure the light is assumed to be polarized in the 

x-direction, which in the C 2v point group can be expressed by a B 1 element. The resulting 

14 A thorough understanding of the transition moment integral 

∫ 

∫ 

M = ψe ∗′ ˆµ e ψ e dτ e or ψ ∗ (r, t)(−er)ψr, t)dV (4.4) 

combined with eqn 2.5 can lead to an extensive evaluation of intensity variations beyond the scope of this 

study [20]. 

15 Although probability is defined as the square of the wavefunction (ψ ·ψ ∗ ). However, for this 2-D diagram 

the positive and negative wavefunctions are drawn as blue and red colors [66].



orbital ψ 

i 

character 

Y (σ v ) 

synchrotron 

polarization 

vector, E & 

Y 

emission ψ 

f 

character 

Y 

= 

X ⊗ 

X 

X 

A 2 

B 1 

B 2 

Z 

Z 

( 1 1 -1 -1 ) ⊗ ( 1 -1 1 -1 ) = ( 1 -1 -1 1 ) 

Z 

(C 2 

) 

Fig. 4.20: An example illustrating the dipole selection rules for an A 2 orbital (found within the 

C 2v symmetry point group) excited by x-polarized light. The mathematic representations are 

shown, along with its pictorial equivalent when described by e.g. the C 2 symmetry axis (lying 

perpendicular to the paper plane) and the mirror plane (coinciding with the y-axis). 

wavefunction, ψ f , again represented mathematically and pictorially must have B 2 character 

as can be directly verified using the C 2v character table 2.5. Graphically, ψ f can be thought 

of as resulting from multiplying the electron probabilities per quadrant. This ψ f is referred 

to as the electron’s emission character because it reveals the detection possibilities for this 

MO. 

To proceed for our spectra comparison, the relationship between symmetry coordinates 

(Figure 4.20) and the experimental coordinate frame must be known. Figure 4.21 relates the 

group theory-based frameworks for all four symmetries to the lab frame. For C 2 , C s , and C 2h 

the z-axis lies collinear to the C 2 axis, thus ascertaining that the two frames are identical 

(right side). Hence, all element character multiplication based purely on these symmetry 

character tables will directly yield electron-detection possibilities for our measured spectra 

in the actual lab coordinates. This does not hold true for the C 2v symmetry shown in 

the left side of Figure 4.21. Here the coordinate systems have exchanged y and z axes 16 ; 

the rotational axis (theoretical z-axis) lies in the experimental sample plane, and the mirror 

plane (theoretical y-axis) has become the axis of detection. This orthogonality of the electron 

emission must be taken into account by transforming one of these frames into the other. 

Hence, returning to the 2-D electron probability cartoon (Figure 4.20), we have to identify 

the y-axis with the (experimental) direction of detection as indicated for our above 

16 Note that the x-axis was unaffected by the translation of the C 2v symmetry coordinates into the lab 

frame, thus any predictions made with respect to the x-axis will also remain unchanged.



Y 

Z 

 

Z 

S 

Experimental Frame 

 

Y 

S 

Experimental Frame 

Z 

Z 

X 

X 

X 

X 

Y 

, C , C 2v 2 s 

Molecular Frame Molecular Frame 

C 2v 

Y 

 

S 

 

S 

C 2h 

Fig. 4.21: Sketch of each symmetry’s coordinates with respect to the experimental framework. 

Notice that the C 2 , C s , and C 2h symmetries are described by coordinates that are identical to the 

lab. However, the y- and z-axes are flipped for C 2v . 

example, i.e. A 2 character together with x-polarized light, in the highlighted top-left corner 

of Figure 4.22. Thus, for this particular geometry one expects ψ f detection in normal electron 

emission in the lab frame (whereas no emission would have been expected for the same 

B 2 orbital when detection lied above the node at the molecular z-axis). This procedure is 

repeated for the two sketches underneath for the same A 2 character excited, however, by y- 

and z-polarized light, respectively. As a reminder, under C 2v symmetry A 2 is the character 

of the HOMO, yet HOMO-1 has B 1 character. Since these are the two orbitals possessing 

the strongest angle dependencies in the PES (Figure 4.13, 4.19), it is worth additionally 

investigating the analogous set of probability sketches for B 1 character, shown on the right 

side of Figure 4.22. 

With the aid of Figure 4.22 it is clear what detection possibilities exist for these two 

MO’s depending on its orientation. We can further summarize the expected emissions for all 

molecular orbitals (for all symmetry groups) and classify them according to photoemission 

occuring in (1) normal direction, N, (2) within a given plane, +, or (3) in fact forbidden, -. 

Such a compilation is presented in Tables 4.4 - 4.7, where we show the respective actual (lab 

frame) results for the most relevant symmetry groups as motivated in Sec. 4.3.2. This is 

done for our main experimental geometries (see Figure 4.12), and the results can be directly 

compared to the spectra in Figure 4.13. 

Examining first the C 2v symmetry, we see dipole rules that fairly well explain the PE 

spectra intensity variations shown in Fig 4.19. X-polarized light invokes an electron emission



Direct Products for C 2v symmetry selection 

Y 

Y 

 

Y 

SR=E x 

Y 

Y 

 

Y 

X 

⊗ X = X 

A 2 

Y 

X 

A 2 

B 1 

Y 

⊗ X = X 

B 2 

 

B 2 B 1 

Y 

X 

⊗ X = X 

B 1 B 2 A 2 

B 1 

A 1 

SR=E y 

Y 

Y 

Y 

X 

⊗ X = X 

 

B 1 

B 1 

X 

A 2 

Y 

Y 

 

⊗ X = X 

X 

A 1 A 2 

SR=E z 

Y 

Y 

Y 

⊗ X = X 

 

A 1 B 1 

Y 

HOMO 

HOMO-1 

Fig. 4.22: Sketch of all the products of the dipole selection rules for the HOMO (A 2 ) and HOMO- 

1 (B 1 ) with C 2v symmetry transformed into the lab frame. The polarization direction of the 

synchrotron light, E, is labeled along the middle of the diagram. 

C 2v MO E x D x E y D y E z D x E z D y E y D x E x D y 

A 1 N N N N N - 

A 2 N - + - + N 

B 1 N - + - + N 

B 2 + N N N + - 

Tab. 4.4: A summary of the symmetry selection rules for C 2v in the lab frame. Here all six 

experimental geometries posses individual selection rules; N stands for allowed in normal emission, 

and plus/minus means allowed (off-normal)/forbidden. 

that would be observable in normal emission for the top of the π-band (HOMO, HOMO- 

1, and HOMO-2, with A 2 , B 1 , and A 2 character, respectively), whereas for these same 

orbitals the transition is forbidden for y-polarized light. Furthermore, when considering the 

respective spectra measured at 20 ◦ off-axis (see figure 4.13), we again observe an allowed 

emission for E x D x but a forbidden one for E y D y , thus re-confirming the C 2v selection rules’ 

expectations (see Table 4.2). Of course, a strictly forbidden transition should yield no PE 

signal, which is not the case here. This failure may be largely attributed to structural 

distortions and broken molecules in the film and at the film’s surface. Recall that broken 

molecules posses no symmetry (see Table 2.1) and hence emission can never be forbidden (by



C 2v symmetry 

0.0328 eV 

0.1086 eV 

0.1095 eV 

0.1872 eV 

Fig. 4.23: Sketch of the four possible 6T conformers having C 2v symmetry. The calculated 

potential energies of the various conformers is (again) displayed on the right, which are shown with 

respect to 0.0 eV all-trans (bulk) conformer. 

symmetry), thus some signal always arises (from these isomers) regardless of the experimental 

set-up. However, one can argue that the small E y D y signal is constant for all ”forbidden” 

transitions (since it arises from the same C 1 -structures). This holds true even for E z D x 

(Figure 4.13, lower box), in which the HOMO and HOMO-1 are not observable in this 

geometry (NE), and likewise show the same intensity as the E y D y ”forbidden” features. 

There are four 6T conformers which possess C 2v symmetry as shown in Figure 4.23. 

Clearly the all-cis conformer (bottom) is energetically and structurally unlikely. However, 

the top cis-conformer has a potential energy of only 0.03 eV (relatively) higher than the most 

favorable all-trans conformer (which is considered here to be 0.0 eV). Additionally, a potential 

energy surface was mapped for a 6T molecule where only the inner rings twist in torsion 

angle between all-trans to the top cis-conformer. The results mirrored those of Figure 4.15 

for bithiophene. In summary, pinpointing the actual cis-conformer would seem impossible 

without measurements, e.g. X-ray diffraction, aimed solely at structural information. 

Contrasting this to the expected emission for C 2h symmetry (Tabel 4.5), the assumed 

bulk structure, we would expect a different emission behavior for the HOMO and HOMO-



C 2h MO E x D x E y D y E z D x E z D y E y D x E x D y 

A g + + N N + + 

A u - - - - - - 

B g N N + + N N 

B u - - - - - - 

Tab. 4.5: A summary of the symmetry selection rules for C 2h . Here all six experimental geometries 

posses individual selection rules; N stands for allowed in normal emission, and plus/minus means 

allowed (off-normal)/forbidden. 

S 

S 

S 

S 

C s 

S 

S 

C 2h 

S 

S 

S 

S 

S 

S 

Fig. 4.24: Sketch of the individual dipoles for 6T confermers with an example of C s symmetry 

(top) and the all-trans 6T (bottom). The net dipole effect of the thiophene monomers for C s is 

not zero, whereas it fully cancels for the all-trans conformer. 

1, specifically that the latter signal should never be observed in photoemission (forbidden 

transition), irrespective of the setup. Since the HOMO, HOMO-1, and HOMO-2 signals are 

always of the same intensity, we thus rule out C 2h as likely to exist as a conformer structure 

at the surface. This same argument can be applied as well to the C 2h ’s reduced symmetry 

state (Table 4.6), C 2 . Here the alternating character of the HOMO and HOMO-1, B and 

A, respectively, should lead to some change of the PE signal for the neighboring orbitals, 

which is not the case here. Additionally, a symmetry reduction to C 2 , with an exact tilt of 

the rings out of plane, would seem unlikely since the molecules have been shown to adopt 

an almost-planar configuration [41]; note that the HB structure is also planar (see Sec 2.3). 

Another symmetry that may be considered is C s , which is a reduced C 2v symmetry. Here 

C 2 MO E x D x E y D y E z D x E z D y E y D x E x D y 

A + + N N + + 

B N N + + N N 

Tab. 4.6: A summary of the symmetry selection rules for C 2 . Here all six experimental geometries 


allowed (off-normal)/forbidden.



C s MO E x D x E y D y E z D x E z D y E y D x E x D y 

A’ + + N N + + 

A” N N + + N N 

Tab. 4.7: A summary of the symmetry selection rules for C s . Here all six experimental geometries 


allowed (off-normal)/forbidden. 

the π-orbitals all have the same character (Table 4.7) leading to identical changes of HOMO 

and HOMO-1 (neither being forbidden), hence correctly accounting for our PE spectra. 

Not as obvious is the explanation for the larger signal observed for x-polarized light. Yet, 

one may alternatively argue that the relative orientation of dipoles of individual thiophene 

units, with respect to the high symmetry axes, needs to be taken into account. Looking 

at the relative arrangement of the 6T monomer units within any conformer (there exists 

twelve) of C s symmetry one notices that the net dipole is non-zero (Figure 4.24). Since a 

thiophene monomer possesses C 2v symmetry, this high-symmetry mirror axis could be then 

excited: thus each monomer unit affected in this way would ”add up” yielding in a larger PE 

signal in only one direction 17 (that of the high-symmetry axis excitation). Contrarily, any 

individual dipoles for any conformer under C 2h symmetry would fully cancel (Figure 4.24, 

bottom). Hence only the C s symmetry would result in this additive PE signal (along the 

chain), which could give rise to the larger top π-features in both the E x D x and the E z D x 

(off-NE) geometries. 

In conclusion, our analysis would strongly suggest the C 2v being the favorable symmetry 

of 6T molecules within the present films. However, additional symmetries, C 1 or C s , seem 

likely to coexist, and moreover, different symmetries may well apply for bulk vs surface 6T’s. 

Furthermore even a herringbone (HB) structure, demonstrated for bulk 6T crystals, could 

still be in fact assumed for the C 2v molecules. Such a packing would not alter the conclusions 

of this symmetry analysis, since this would imply only a rotation around the x-axis, which 

in our case is a low-symmetry plane. Note, then that off-axis measurements that examine 

the x-z plane are not suited to reveal forbidden transitions. 

17 The presence of the sulfur atom within the rings breaks the symmetry across the x-axis, thus reducing 

this plane to a low symmetry axis.

Chapter 5 

Conclusions 

Ordered sexithiophene (6T) thin films, grown on a Au(110) single-crystal surface, were studied 

by angle-resolved photoemission using EUV photon energies. The observed pronounced 

signal intensity variations as a function of the experimental geometry were analyzed in terms 

of the orientation of the 6T molecules with respect to the substrate’s principle axes. The 

films were produced by evaporation of 6T in ultra-high vacuum. Structural analysis, up to 

a few layers thickness, was inferred by LEED, and the morphology of thicker films (ca. 2000 

Å), which were of primary interest here, was characterized by AFM. The latter films consisted 

of islands with a preferred order paralleling that of the crystals’ main axes. Varying 

the sample’s azimuth, the photon incidence (including orientation of the light polarization 

vector), and the electron detection angle allowed different experimental excitation/detection 

geometries to be realized. Since the films studied were rather low conducting, upon photoelectron 

emission, positive charge accumulates at the surface. In order to achieve sharp 

spectral features, and hence to enable for a quantitative analysis of the observed emission 

anisotropies, charge was fully compensated using simultaneous laser radiation. 

Information on the order and orientation of the 6T molecules within these films respect 

was inferred through symmetry selection rules. To accomplish this, quantum-chemical calculations 

(HF/G-31(d,p)) were used, in a first step, to calculate the orbital energies for several 

possible conformers, each optimized for a particular symmetry group. The results indicated 

that the total energy of the C 2h (trans) and C 2v (cis) conformers had a relative (potential 

energy) difference of only 0.03 eV, thus rendering it reasonable to assume the presence of 

several conformers in our films. In fact, we do not necessarily expect identical conformers as

80 

for bulk crystalline 6T; in addition we need to consider both surface and bulk contributions 

to the spectra, since for the photon energy used here (typically 50 eV) the technique is quite 

surface specific. Additionally calculated was each conformer’s orbital character, determined 

by the symmetry group, which is crucial for assigning allowed (observable) or forbidden 

transitions associated with each experimental geometry. Here we have mainly considered 

the changes within the π-orbital spectral range, particularly HOMO and HOMO-1 intensity 

variations. Then, applying group theory and symmetry-derived selection rules to the various 

possible (optimized) conformers, we arrived at the conclusion that C 2v and C s are the most 

likely molecular symmetries in the films. This contrasts from C 2h corresponding to bulkcrystalline 

6T. Our findings are consistent with the 6T molecules being aligned with the long 

molecular axis in the direction of the [110] troughs. The work presented here contributes 

to our knowledge of ordered sexithiophene thin films clearly evidencing the importance of 

structural anisotropy on physical properties.

Chapter 6 

Appendix 

6.1 Laser Systems Used in Combination with Synchrotron 

Light 

The following is a brief description of the laser systems available at that time at the MBI User 

Facility (where the presented experiments were performed). This facility is dedicated to the 

investigation of the dynamics of photon-induced processes within various surface molecular 

systems, which is accomplished by using time-correlated laser (LR) and synchrotron (SR) 

pulses as sketched in Figure 6.1. Different laser systems, with regard to wavelengths and 

repetition rates (and hence pulse energies), are available to meet different experimental 

requirements. Typically, excitation is done by fs or ps LR pulses (synchronized to either 

single or multi bunch SR pulses), and probing is performed by time-delayed SR pulses (ca. 

30-50 ps SR pulse width). Various processes may be studied by this technique, including 

among others charge carrier dynamics, photoisomerization, transiently excited electronic 

molecular states, phase transitions, and photodissociation. As we have mentioned, for the 

current de-charging experiments synchronization of the LR and SR pulses is not necessarily 

required. 

Figure 6.2 depicts the interpulse spacing of the two laser systems, available for the present 

measurements, relative to the synchrotron repetition rate for multi and single bunch operation 

modes. The top figure shows the 83.3 MHz pulses from the Ti:sapphire laser synchronized 

to the multi bunch (500 MHz) in a ratio of 1:12, and the bottom figure illustrates the 

respective situation for the Nd:YVO 4 laser synchronized (1:1) to single bunch (1.25 MHz). 

This repetition rate is actually achieved by pulse-picking from a 25 MHz Vanadate oscillator

6.1 Laser Systems Used in Combination with Synchrotron Light 82 

Fig. 6.1: Sketch of the principle idea of combining laser and synchrotron pulses, performed at the 

MBI-BESSY User Facility facility. The BESSY multi-bunch operation (500 MHz) is depicted. 

(see center panel). The synchronization in both cases is achieved by an analog phase-locked 

loop (PLL) scheme [50], [67], involving the precise control of the laser cavity lengths. 

SR 

LR 

2 ns 

12 ns 

Ti:sapphire laser 

83 MHz rep. rate (pumped by a 10W Nd:YAG laser), 

200 fs or 4 ps pulse width, 

output power ca. 2 W @ 760-950 nm (fundamental), 

ca. 500 mW (second harmonic, SH); 

synchronized to BESSY multi bunch (500 MHz); 

accuracy better than 5ps 

SR 

LR 

40 ns 

800 ns 

pulse picked 

Time 

Nd:YVO 4 laser 

Low repetition 1.25 MHz rep. rate 

(pulse-picked from 25 MHz oscillator); 

14 ps pulse width, 

ca. 200 mW @ 1064 nm; 

100-250 nJ pulse energy @ 532 nm; 

1:1 synchronized in single bunch (1.25 MHz), 

accuracy better than 5ps 

Fig. 6.2: Time structure of the SR and LR pulses. Different laser systems are used for multi (top) 

and single (bottom) bunch, a Ti:sapphire and a Nd:YVO 4 , parameters for which are described in 

the figure.

List of Figures 

2.1 Illustration of photoemission spectroscopy . . . . . . . . . . . . . . . . . . . 8 

2.2 Universal mean-free path of electrons . . . . . . . . . . . . . . . . . . . . . . 11 

2.3 Schematic of synchrotron undulator . . . . . . . . . . . . . . . . . . . . . . . 12 

2.4 Sketch of the 6T atomic architecture . . . . . . . . . . . . . . . . . . . . . . 14 

2.5 Stereographic 6T unit cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

2.6 Space filling herringbone packing of the solid 6T . . . . . . . . . . . . . . . . 16 

2.7 Absorbtion spectrum of ca. 50nm 6T/quartz . . . . . . . . . . . . . . . . . . 17 

2.8 Electronic structure and character (aromatic vs quinoid) of thiophene rings . 18 

2.9 Examples of the discussed symmetries C 1 , C 2 , C s , C 2h , and C 2v , as illustrated 

in thiophene isomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

2.10 Graphic illustration of anti-bonding and bonding wavefunctions for the H + 2 . 24 

2.11 Graphic illustration of the bonding energy as a function of R ab . . . . . . . . 25 

2.12 Graphic illustration of the sp-hybridization . . . . . . . . . . . . . . . . . . . 26 

2.13 Cartoon depicting the resulting polarization for sp- and pd-hybridization. . . 26 

3.1 Schematic of the undulator (U125/1) MBI-Beamline at BESSY II . . . . . . 28 

3.2 Principle of the U125/1 monochromator spherical gratings . . . . . . . . . . 29 

3.3 Schematic layout of the MBI UHV surface apparatus . . . . . . . . . . . . . 30 

3.4 Photograph of the MBI User Facility at BESSY II . . . . . . . . . . . . . . . 32 

4.1 LEED images of 0, 1, and 2 Å 6T/Au(110) . . . . . . . . . . . . . . . . . . . 37 

4.2 Sketch of the Au(110) surface reconstructions upon low depositions of 6T [39] 38 

4.3 LEED images of clean Au(110) and Au(111) surfaces and low 6T coverages . 40 

4.4 AFM top-view images of thin 6T films grown on both Au(110) and Au(111) 42

LIST OF FIGURES 84 

4.5 Quantitative analysis of certain section cuts of the AFM images . . . . . . . 43 

4.6 X-ray photoemission spectra of increasing 6T coverages . . . . . . . . . . . . 45 

4.7 X-ray photoemission spectra of thin 6T films grown on both Au(110) and 

Au(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 

4.8 Photoemission spectra of 6T films grown on Au(110) with increasing thickness 48 

4.9 Photoemission spectra of ca. 2000 Å 6T/Au(110) with and without the addition 

of laser irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

4.10 Photoemission spectra showing the induced charging of a 6T film as a function 

of synchrotron photon flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

4.11 Charge compensation of photoemission spectra of a 2000 Å 6T film obtained 

at the highest photon flux in combination with various percentages of laser 

light irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

4.12 Sketch of the experimental geometry used in measuring angle-resolved photoemission 

spectra of 6T/Au(110) . . . . . . . . . . . . . . . . . . . . . . . . 57 

4.13 Angle-resolved photoemission spectra of ca. 2000 Å 6T/Au(110) as a function 

of experimental geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

4.14 Schematic drawing of the gas-phase optimized molecular geometry for a bithiophene 

molecule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 

4.15 Potential energy curve of bithiophene conformers . . . . . . . . . . . . . . . 63 

4.16 Schematic drawing of the gas-phase optimized molecular geometry for an individual 

sexithiophene molecule . . . . . . . . . . . . . . . . . . . . . . . . . 65 

4.17 Calculated MO’s for even n-thiophene molecules with simple cartoon depicting 

anti-, non-, and bonding characteristics . . . . . . . . . . . . . . . . . . . . . 66 

4.18 Schematic of the actual molecular orbitals of the π-system of sexithiophene . 68 

4.19 Calculated binding energies for various symmetries directly compared to reference 

angle-resolved PE spectra . . . . . . . . . . . . . . . . . . . . . . . . 71 

4.20 A mathematical and visual example for the application of symmetry selection 

rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 

4.21 Transformation of the symmetry coordinates into the lab frame . . . . . . . 74

LIST OF FIGURES 85 

4.22 Sketch of the HOMO and HOMO-1 dipole selection rules for C 2v symmetry 

transformed into the lab frame . . . . . . . . . . . . . . . . . . . . . . . . . . 75 

4.23 Sketch of the four calculated C 2v conformers . . . . . . . . . . . . . . . . . . 76 

4.24 Sketch of the individual dipole effect between a C s and a C 2h conformer . . . 77 

6.1 Sketch of the principle idea of the MBI-BESSY user facility . . . . . . . . . . 82 

6.2 Time structure of the SR and LR pulses . . . . . . . . . . . . . . . . . . . . 82

List of Tables 

2.1 The C 1 symmetry character table . . . . . . . . . . . . . . . . . . . . . . . . 21 

2.2 The C 2 symmetry character table . . . . . . . . . . . . . . . . . . . . . . . . 21 

2.3 The C s symmetry character table . . . . . . . . . . . . . . . . . . . . . . . . 21 

2.4 The C 2h symmetry character table . . . . . . . . . . . . . . . . . . . . . . . . 22 

2.5 The C 2v symmetry character table . . . . . . . . . . . . . . . . . . . . . . . . 22 

4.1 Summary of the 6T structures’ measured dimensions as observed through AFM 44 

4.2 Summary of the change in relative intensities of the photoemission signal as 

recorded by angle-resolved photoemission . . . . . . . . . . . . . . . . . . . . 58 

4.3 Summary of the calculated MO’s for C 2 , C 2h , C 2v , all-C 2v , and C s symmetries 69 

4.4 Symmetry selection rules for C 2v in the lab frame . . . . . . . . . . . . . . . 75 

4.5 Symmetry selection rules for C 2h in the lab frame . . . . . . . . . . . . . . . 77 

4.6 Symmetry selection rules for C 2 in the lab frame . . . . . . . . . . . . . . . . 77 

4.7 Symmetry selection rules for C s in the lab frame . . . . . . . . . . . . . . . . 78

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Acknowledgements 

I would like to thank to Prof. I. V. Hertel for giving me the opportunity to work at the 

Max Born Institute, and also for his support and interest in my studies. 

I would also like to thank Prof. Wolf Widdra for his helpful discussions and constructive 

comments pertaining to my work. 

I am grateful to Dr. Bernd Winter for his endless support, enthusiasm, encouragement, 

and careful guidance concerning every aspect of my work. I am indebted to him for helping 

me gain a true insight and understanding of organic molecules, photoemission, and other 

surface science techniques, also for providing a friendly and hands-on learning atmosphere 

at BESSY, and for his help and guidance with data and spectra analyzation. He also 

patiently read through many rough drafts of my work and listened to many rehearsals for 

my conference talks and always provided great feedback (even for the english, aber auch für 

deutsch). Additionally and essentially, he made physics fun. 

I would also like to acknowledge Prof. W. Raith for his kind advice and interest in my 

activities here at MBI. 

A very special thanks goes to Dr. Jens Dreyer. His incredible guidance and friendly 

nature allowed me to not only understand and implement quantum-chemical calculations 

into my work, but even enjoy it. He was always there when I needed him and quickly 

responded to any questions I had. 

I would also like to thank Dr. H.-H. Ritze for his very interesting and detailed discussions 

into the theoretical side of my work. Additionally, his comments regarding group theory and 

symmetry selection rules helped me significantly in understanding my results. 

I must heartily thank Dr. Norbert Koch for sharing his immense knowledge about organic 

molecules, and really, really good coffee, with me. He gave me a thorough understanding 

about my topic and advised me in several aspects concerning my studies and my writing. 

He also taught me invaluable research lessons, when I had the pleasure of measuring with 

him, and always encouraged and respected my ideas; he was to me not only a colleague but 

a good friend and a wonderful teacher as well.

Special thanks goes to Dr. Sigurd Schrader, Prof. Guglielmo Lanzani, and Prof. Riccardo 

Tubino for their encouragement and motivation outside the institute. Additionally I would 

like to thank Prof. Kazuhiko Seki, Dr. Thorsten Kampen, and Prof. Michael Ramsey for 

their email correspondence and thoughtful answers to my questions. 

I also would like to express my gratitude to Dr. Wolfgang Freyer, who performed the 

optical absorption measurements. 

Also I am grateful to Dr. Diana Pop for her patient help in many aspects of my work 

and the warm environment she provided here at MBI. She always took the time to share her 

extensive knowledge of not only organic systems, but also problems pertaining to BESSY, 

computers, network, and foreigner issues. I had a lot of fun together with her at the DPG 

in Regensburg and highly value her opinion and our friendship. 

A special thanks goes to Helena Prima-Garcia for her optimism and contagious good 

mood. I am so happy we started together at MBI, she often encouraged and helped me (she 

also showed me how to really cook) along the way. 

I would like to thank Reinhard Grosser for his support in taking care of the UHV chambers 

and other technical activities in BESSY, and especially for always helping out even if things 

were last minute. I also very much appreciate my time working with Roman Peslin. I really 

enjoyed working with him in his workshop on my sample probe and could always count on 

him for great work and ingenuity, and a good laugh. 

I also like to express my appreciation to my colleagues, first Dr. Tanja Giessel and 

David Bröcker for their assistance in many physics, BESSY, and computer issues, specifically 

Tanja’s help in Igor and David’s thorough tutorials introducing me to LaTeX. I also 

appreciate Dr. Ramona Weber and Philipp Martin Schmidt for all the fun in my water 

measuring time in my first year. Philipp’s passion for physics and his unwavering offer to 

help really makes him one of a kind and a pleasure to work with. 

I would also like to thank Rainer Schuman, Thomas Kruel, and Michael Dose for their 

expert and fast aid with lasers, computers, and monochromators, respectively. Many thanks 

also to the whole team in Haus A for the friendly environment at MBI. Also, I specifically 

appreciate Mr. B. Kinski, Mrs. K. Lekve, and Mrs. Sabine Winter for their help in solving 

all administrative issues.

Additionally, I send a lot of love and thanks to my boyfriend (SV) and all my friends 

I’ve made here in Berlin who have helped me to make a home here, taught me German, and 

just kept me grounded during my studies. 

Last, but certainly not least, I would like to especially thank my parents, my aunt and 

grandma, my sister, and those few truly great friends for all of their love, support, longdistance 

telephone conversations and lengthy emails, and the general great feeling I have 

from just having you all in my life. Thanks for believing in me and helping me believe in 

myself.

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