This thesis is based on a selection of six different experimental techniques used for studies in atomic and molecular physics. The techniques analysed in the thesis are compared to find similarities in strategies and ways to avoid sources of error. Paper 1 deals with collision based spectroscopy with 60 keV Xe6+ ions on sodium and argon gas targets. Information on energy of Rydberg states in Xe5+ is unveiled by optical spectroscopy in the wavelength range from vacuum ultraviolet (VUV) to visible. In paper 2, the fast ion-beam laser spectroscopy (FIBLAS) is adapted for measuring hyperfine structure of barium isotopes in an isotopically pure ion-beam. This techniques involves changing the isotope during the measurement to minimize sources of error in measurement and enhance the signal from lesser abundant isotopes. The FIBLAS technique is used in paper 3 to study samarium ions. The ions are optically pumped and the recorded optical nutation is used to measure transition probabilities. This technique eludes the difficulties inherent in relative intensity measurements of all the radiative transitions from an excited state. In Paper 4, optical emission spectroscopy is used in the VUV region to study noble gas mixture discharges. The source of the emission bands near the resonance lines of krypton and xenon are found to be heteronuclear dimers. In paper 5, radiation from a pulsed argon plasma with admixture of nitrogen is studied with time resolved spectroscopy in the VUV and ultraviolet wavelength ranges to investigate the mechanism of energy transport. A metastable state of atomic argon is found to be an important source of energy to many radiative processes. In Paper 6, photoelectron spectroscopy (PES) on thiophene, on 3-bromothiophene and on 3,4-dibromothiophene using time-of-flight photoelectron-photoelectron coincidence technique and conventional PES to investigate the onset of double ionisation compared to the onset of single ionisation in molecules.

Abstract In this work an experimental study of size varied, neutral, and free metal clusters using synchrotron radiation excited photoelectron spectroscopy was performed. The combined core-level and valence photoelectron spectroscopic investigation indicates metallic properties for nanoscale Rb, K, Sn, and Bi clusters. In the case of Sn the experimental results suggest a metal-to-insulator transition occurring at the studied size range. In addition to the experimental results the technical implementation of the cluster production set-up is presented and jellium-model-based simulations are compared with the experimental results of the Rb and K clusters.

A novel strategy for the intercalation of antimony (Sb) under the (6√3 × 6√3)R30° reconstruction, also known as buffer layer, on SiC(0001) is reported. Using X-ray photoelectron spectroscopy, low-energy electron diffraction, and angle-resolved photoelectron spectroscopy, it is demonstrated that, while the intercalation of the volatile Sb is not possible by annealing the Sb-coated buffer layer in ultrahigh vacuum, it can be achieved by annealing the sample in an atmosphere of Ar, which suppresses Sb desorption. The intercalation leads to a decoupling of the buffer layer from the SiC(0001) surface and the formation of quasi-freestanding graphene. The intercalation process paves the way for future studies of the formation of quasi-freestanding graphene by intercalation of high-vapor-pressure elements, which are not accessible by previously known intercalation techniques, and thus provides new avenues for the manipulation of epitaxial graphene on SiC.

The rechargeable Li-O2 battery has attracted interest due to its high theoretical energy density (about 10 times better than today’s Li-ion batteries). In this PhD thesis the cycling instability of the Li-O2 battery has been studied. Degradation of the battery has been followed by studying the interface between the electrodes and electrolyte and determining the chemical composition and quantity of degradation products formed after varied cycling conditions. For this in-house and synchrotron based Photoelectron Spectroscopy (PES) were used as a powerful surface sensitive technique. Using these methods quantitative and qualitative information was obtained of both amorphous and crystalline compounds. To make the most realistic studies the carbon cathode pore structure was optimised by varying the binder to carbon ratio. This was shown to have an effect on improving the discharge capacity. For Li-O2 batteries electrolyte decomposition is a major challenge. The stability of different electrolyte solvents and salts were investigated. Aprotic carbonate and ether based solvents such as PC, EC/DEC, TEGDME, and PEGDME were found to decompose during electrochemical cycling of the cells. The carbonate based electrolytes decompose to form a 5-10 nm thick surface layer on the carbon cathode during discharge which was then removed during battery charging. The degradation products of the ether based electrolytes consisted mainly of ether and carbonate based surface species. It is also shown that Li2O2 as the final discharge product of the cell is chemically reactive and decomposes carbonate and ether based solvents. The stability of lithium electrolyte salts (such as LiPF6, LiBF4, LiB(CN)4, LiBOB, and LiClO4) was also studied. The PES results revealed that all salts are unstable during the cell cycling and in contact with Li2O2. Decomposition layers thinner than 5 nm were observed on Li2O2. Furthermore, it is shown that the stability of the interface on the lithium anode is a chief issue. When compared to Li batteries (where oxygen levels are below 10 ppm) working in the presence of excess oxygen leads to the decomposition of carbonate based electrolytes to a larger degree.

Understanding the fundamental electronic properties of materials is a key step to develop innovations in many fields of technology. For example, this has allowed to design molecular based devices like organic field effect transistors, organic solar cells and molecular switches. In this thesis, the properties of advanced functional materials, in particular metal-organic molecules and molecular building blocks of 2D materials, are investigated by means of Density Functional Theory (DFT), the GW approximation (GWA) and the Bethe-Salpeter equation (BSE), also in conjunction with experimental studies. The main focus is on calculations aiming to understand spectroscopic results. In detail, the molecular architectures of lutetium-bis-phthalocyanine (LuPc2) on clean and hydrogenated vicinal Si(100)2×1, and of the biphenylene molecule on Cu(111) were analysed by means of X-ray Photoelectron spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy; DFT calculations were performed to obtain insights into the atomic and electronic structures. Furthermore, detailed information about the electronic states of the gas phase iron phthalocyanine (FePc) and of the gas phase biphenylene molecule were obtained through XPS and NEXAFS spectroscopy. I have studied by means of DFT, multiplet and GWA calculations the electronic correlation effects in these systems. Also the optical, electronic and excitonic properties of a hypothetical 2D material based on the biphenylene molecule were investigated by GWA and BSE calculations. Monolayers of metal-free phthalocyanine (H2Pc) on Au(111) and of FePc on Au(111) and Cu(100)c(2×2)-2N/Cu(111) with and without pyridine modifier were studied by XPS and final state calculations. A multiplet approach to compute L-edges employing the hybridizations function, known from dynamical mean field theory, was proposed and applied to transition metal oxides.

In disordered alloys, atoms belonging to the same chemical element will exhibit different environments. This leads to variations in the atoms’ local electronic structures, which in turn leads to variations in the binding energies of their core levels. These binding energies can be measured experimentally using core level X-ray photoelectron spectroscopy (XPS). Therefore, in theory at least, core level XPS can be used to resolve different environments in alloys. However, to make this a reality one must understand how an atom’s local electronic structure, and hence the binding energies of its core levels, are affected by local environment. In this thesis, two simple phenomenological models are explored which purport to correctly describe the local electronic structure of disordered alloys. The first model which we consider has its roots in chemical intuition; specifically, the notion that pairs of unlike atoms, i.e. atoms belonging to different chemical elements, transfer a certain quantity of charge, while like atoms do not. Using this model - known as the optimised linear charge model (OLCM) - the relationship between an atom’s local electronic structure, core level binding energies, and its environment is explored in detail, both in the bulk of disordered alloys and near their surfaces. As well as ‘homogeneous’ disordered alloys, in which the concentrations of the alloy’s constituent elements are the same throughout the entire alloy, various ‘inhomogeneous’ disordered alloy systems are considered. These include alloys exhibiting surface segregation - in which the concentrations at the surface differ from those in the bulk - as well as interfaces between two metals with various levels of intermixing. The results of our investigation of bulk inhomogeneous alloys are compared to analogous ab initio results, which confirms the model’s viability as a tool for rationalising the relationship between local electronic structure, core level binding energies, and environment. More generally, our results also reveal a number of interesting new phenomena. Firstly, the widths of spectra in inhomogeneous disordered alloys are significantly larger in some cases than is possible in any analogous homogeneous disordered alloy. Secondly, differences between the concentrations of each element at the surface and deep within the bulk cause a shift in the work function of the alloy under consideration. The latter results in qualitatively different trends than one would expect if this phenomenon was ignored, and prompts an alternative interpretation of the results of a recent experimental study. The second model which we consider is a particular case of the charge-excess functional model, in which the realised charges on all atoms are those which minimise a particular expression for the total energy of the system, and whose accuracy has been well established. The underlying assumptions and properties of this model are explored in detail, adding insight into the nature of the screening and inter-atomic interactions in disordered alloys. The model is shown to be equivalent to the OLCM for the case of binary alloys, and can therefore be considered to be the generalisation of the OLCM for alloys containing more than two chemical elements. The model is also used to derive analytical expressions for various physical quantities for any alloy, including the width of core level XPS spectra and the Madelung energy. These expressions are then used to investigate how the physical quantities to which they pertain vary with the concentrations of each element in a homogeneous disordered alloy consisting of three elements. Among other things, it was observed that the width of the core level XPS spectra is maximised when the concentrations of the two elements in the alloy with the largest electronegativity difference have equal concentrations, while the remaining element has a vanishing concentration.

Transition metal-ligand interactions in several groups of closely related organometallic complexes are discussed from the results of both valence and core photoelectron experiments. Particular attention is given to the novel experimental aspects, including a charged particle oscillator He II source, sample introduction and containment, and data collection and spectral analysis procedures not normally associated with gas phase photo-electron spectroscopy. The application of the ionization experiments begins with a reassessment of the bonding in the group VIb metal hexacarbonyls. He I ionization data of unprecedented quality for the predominantly metal d t₂g level of Cr(CO)₆ and W(CO)₆ reveals for the first time the presence of metal-carbon vibrational fine structure. These positive ion M-C stretching frequencies are significantly reduced from neutral ground state values, giving direct evidence of the pi back-bonding nature of the t₂g level. The next chapter focuses on the comparison of the metal-nitrosyl interactions in the trans-X-W(CO)₄NO complexes to the isoelectronic/isostructural metal-carbonyl interactions in the X-Re(CO)₅ complexes (X = Cl,Br,I). A further comparison of carbonyl and nitrosyl bonding, as well as the first photoelectron assessment of metal-thionitrosyl bonding, is addressed in the next chapter by comparing the valence and core ionization data for CpCr(CO)₂NO and CpCr(CO)₂NS (Sp = η⁵-C₅H₅) to the data reported earlier for CpMn(CO)₃ and CpMn(CO)₂CS. The final chapter of the dissertation compares the electronic structure of the CpFe(CO)₂X complexes to their CpCr(NO)₂X analogs (X = Cl,Br,I,CH₃,CN). The essence of this work fully contrasts the Fe(CO)₂ and Cr(NO)₂ functional groups.

The studies presented in this work are aimed towards a better understanding of the fundamental physics of the electrode/organic molecule interface in both the ground and excited state manifolds. Systematic investigations of single systems using two-photon photoemission (TPPE) and ultraviolet photoelectron spectroscopy (UPS) were undertaken in order to assess the evolution of the electronic structure and molecular organization at the interface. The adsorbate molecule vanadyl naphthalocyanine (VONc) was used whose properties are well-suited to this purpose. Interfacial electronic states of thin films of VONc were studied with two different substrates: highly ordered pyrolytic graphite (HOPG) and Au(111).The substrate of HOPG is a surface which does not possess reactive dangling bonds and the electron density close to the Fermi edge is very low, permitting high resolution spectroscopic band analysis of VONc and revealing subtle changes to the electronic structure. From interfacial studies of this weakly interacting substrate/ adsorbate system, it is shown in this work that molecular electronic levels in both the ground and excited state manifolds can shift independently of the vacuum level. Further, electron transfer between close lying electron donor and acceptor energy levels may be influenced by energy level shifts caused by depolarization effects as a function of dipole density.The VONc/Au(111) interface is investigated in order to examine energy level alignment in a system with the additional complexity of molecule/substrate interactions. The electron rich Au(111) surface leads to a strong interface dipole upon addition of VONc. Joint experimental and computational data is presented showing that the underlying cause of this interface dipole is Pauli repulsion. Additionally, investigations of energy level alignment in the excited state manifold are presented and the possibility of quantum interference is discussed.The interfacial electronic structure is quite different among these two model systems. The interfacial alignment observed in the HOPG/VONc system was largely due to depolarization of the intrinsic molecular dipole as a function of density, whereas the Au(111)/VONc interface is dominated by interfacial Pauli repulsion interactions.

The localized model of a chemical bond has had a long and prominent role in chemistry, but situations of extended charge delocalization and dipole effects remain topics in need of greater understanding. Both orbital delocalization in isolated molecules and induced molecular dipoles in condensed phases serve to move electron density and influence the chemical and physical properties of a system. This dissertation studies these aspects of electronic structure for selected organic, inorganic, and organometallic systems by means of electronic structure calculations and photoelectron spectroscopy, which is well-suited for studying both intramolecular and intermolecular effects by providing a direct probe of orbital energies and characters. Photoelectron spectra of P₄ and AsP₃ reveal differences in the molecular symmetry and cationic state effects between the two molecules in Chapter 3. Despite these differences, AsP₃ is found to have electron delocalization and vibrational structures that are comparable to P₄. A similar study of the delocalized -system of 2H-1,2,3-triazole in Chapter 4 relates the vibrational structure in photoelectron spectroscopy data to a series of Rydberg excitations in the vacuum UV photoabsorption spectrum. Chapters 5 and 6 examine extended electronic structures in organometallic complexes. The electron delocalization and charge transfer between two Ru centers along a bridging ethynediyl ligand is studied in [CpRu(CO)₂]₂(μC≡C). Details of the Ru-alkynyl interaction were explored by comparing the spectra of CpRu(CO)₂C≡CMe with CpRu(CO)₂Cl, including the -backbonding ability of alkynyl ligands. Chapter 6 moves from the realm of intramolecular effects to intermolecular interactions to understand how surrounding media affect electronic properties of molecules. The reversal of ionization energies between the gas and solid phases of M(CO)₄dmpe and M(CO)₄dppe, where M = Mo, W, is explored with photoelectron spectroscopy. The surrounding molecular environment stabilizes the cation, resulting in this reversal that extends to core ionization energies. The variety of systems presented illustrates the wide applicability of photoelectron spectroscopy and computations to different electronic structure studies, including how gas phase results can be related to condensed phase studies. This work continues the progress of photoelectron spectroscopy from small molecules to larger molecular systems and even further to bulk systems.

Interfacial charge transfer between metal oxides and organic semiconductors has been found to limit the efficiency of organic optoelectronic devices. Although a number of investigations of inorganic/organic systems exist, very few generally applicable rules for oxide/organic interfaces have been developed and many questions about these systems remain unanswered. Thus the studies presented in this dissertation were designed to improve the understanding of the fundamental interface physics of metal oxide/organic systems. Single molecule fluorescence microscopy was employed to determine the charge transfer mechanism while photoelectron spectroscopy was used to determine the energy level alignment of model systems. Additional computational studies allowed the examination of the properties of the charged organic molecules involved in charge transfer and modeling of the molecule-surface interaction. Calculations of the ground state properties and excited state transitions of the neutral and singly charged states of a modified perylene molecule were performed to provide insight into the orbitals of the initial and final states involved in the interfacial charge transfer process. The design and implementation of a novel UHV single molecule microscope is described. This microscope was used to observe the excited state charge transfer between a modified perylene molecule and Al₂O₃ (0001). The charge transfer mechanism was identified as involving activated trapping and detrapping of the defect derived states within the Al₂O₃ band gap, which resulted in the observation of strongly distributed kinetics for this system. The influence of defects and adsorbates on the electronic structure of ZnO and its interface with organic semiconductors was determined from photoelectron spectroscopy. Modified perylene molecules were found to have strong chemisorptive interactions with the ZnO surface involving charge transfer from defect derived ZnO states to the LUMO, while magnesium phthalocyanine molecules appear to have only weak physisorptive interactions with the ZnO surface. The interfacial investigations of the organic/oxide systems demonstrate the rich defect structure present in metal oxides. In both cases, defects were found to control the interfacial interactions between the metal oxide surface and the modified perylene molecules. Thus the manipulation of these defects states is of fundamental importance for optoelectronic device design.