<p>Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis.</p><p>In the case of alkanes, <i>n</i>-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in <i>n</i>-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and <i>n</i>-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in <i>n</i>-octane are shortened upon adsorption and the C-H bonds are elongated in both <i>n</i>-octane and methane.</p><p>In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds.</p><p>The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. </p><p>Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp².</p>

Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis. In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane. In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds. The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.

The present thesis investigates the electronic and structural properties of adenine, cytosine, and guanine layers on hydrogen passivated silicon (111)(7x7). The (7x7) reconstruction of the silicon surface was achieved by direct current heating of the samples in UHV conditions. After in situ hydrogen passivation layers of the DNA bases were prepared in different thicknesses by means of organic molecular beam deposition, all samples were characterized employing valence band and core level photoemission spectroscopy. Additionally the near edge x-ray absorption fine structure of the DNA base layers was investigated. A detailed and consistent picture of structural and electronic properties of the nucleotide bases in the solid state could be developed by comparison of measurements and DFT/B3LYP calculations.

<p>The structure, bonding and chemistry of water and hydroxyl on metal surfaces are presented. Synchrotron based x-ray photoelectron- and x-ray absorption spectroscopy along with density functional theory calculations mainly form the basis of the results. Conditions span the temperature range 35 - 520 K and pressures from ultra-high vacuum (~10 fAtm) to near ambient pressures (~1 mAtm). The results provide, e.g, new insights on the importance of hydrogen bonding for surface chemical kinetics.</p><p>Water adsorbs intact on the Pt(111), Ru(001) and Cu(110) surfaces at low temperatures forming 2-dimensional wetting layers where bonding to the metal (M) mainly occurs via H₂O-M and M-HOH bonds. Observed isotope differences in structure and kinetics for H₂O and D₂O adsorption on Ru(001) are due to qualitatively different surface chemistries. D₂O desorbs intact but H₂O dissociates in kinetic competition with desorption similar to the D₂O/Cu(110) system. The intact water layers are very sensitive to x-ray and electron induced damage.</p><p>The mixed H₂O:OH phase on Ru(001) consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH decorates the edges of the stripes. On Pt(111), two different long-range ordered mixed H₂O:OH structures are found to be inter-related by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H₂O.</p><p>Water adsorption on Cu(110) was studied at near ambient conditions and compared to Cu(111). Whereas Cu(111) remains clean, Cu(110) holds significant amounts of water in a mixed H₂O:OH layer. The difference is explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of strong H₂O-OH bonds. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.</p>

<p>Ordinary yet unique, water is the substance on which life is based. Water seems, at first sight, to be a very simple molecule, consisting of two hydrogen atoms attached to one oxygen. Its small size belies the complexity of its action and its numerous anomalies, central to a broad class of important phenomena, ranging from global current circulation, terrestrial water and CO₂ cycles to corrosion and wetting. The explanation of this complex behavior comes from water's unique ability to form extensive three-dimensional networks of hydrogen-bonds, whose nature and structures, in spite of a great deal of efforts involving a plethora of experimental and theoretical techniques, still lacks a complete scientific understanding.</p><p>This thesis is devoted to the study of the local structure of hydrogen-bonded liquids, with a particular emphasis on water, taking advantage of a combination of core-level spectroscopies and density functional theory spectra calculations. X-ray absorption, in particular, is found to be sensitive to the local hydrogen-bond environment, thus offering a very promising tool for spectroscopic identification of specific structural configurations in water, alcohols and aqueous solutions. More specifically, the characteristic spectroscopic signature of the broken hydrogen-bond at the hydrogen side is used to analyze the structure of bulk water, leading to the finding that most molecules are arranged in two hydrogen-bond configurations, in contrast to the picture provided by molecular dynamics simulations. At the liquid-vapor interface, an interplay of surface sensitive measurements and theoretical calculations enables us to distinguish a new interfacial species in equilibrium with the gas. In a similar approach the cluster form of the excess proton in highly concentrated acid solutions and the different coordination of methanol at the vacuum interface and in the bulk can also be clearly identified.</p><p>Finally the ability of core-level spectroscopies, aided by sophisticated density functional theory calculations, to directly probe the valence electronic structure of a system is used to observe the nature of the interaction between water molecules and solvated ions in solution. Water around transition metal ions is found to interact with the solute via orbital mixing with the metal d-orbitals. The hydrogen-bond between water molecules is explained in terms of electrostatic interactions enhanced by charge rehybridization in which charge transfer between connecting molecules is shown to be fundamental.</p>

Ordinary yet unique, water is the substance on which life is based. Water seems, at first sight, to be a very simple molecule, consisting of two hydrogen atoms attached to one oxygen. Its small size belies the complexity of its action and its numerous anomalies, central to a broad class of important phenomena, ranging from global current circulation, terrestrial water and CO2 cycles to corrosion and wetting. The explanation of this complex behavior comes from water's unique ability to form extensive three-dimensional networks of hydrogen-bonds, whose nature and structures, in spite of a great deal of efforts involving a plethora of experimental and theoretical techniques, still lacks a complete scientific understanding. This thesis is devoted to the study of the local structure of hydrogen-bonded liquids, with a particular emphasis on water, taking advantage of a combination of core-level spectroscopies and density functional theory spectra calculations. X-ray absorption, in particular, is found to be sensitive to the local hydrogen-bond environment, thus offering a very promising tool for spectroscopic identification of specific structural configurations in water, alcohols and aqueous solutions. More specifically, the characteristic spectroscopic signature of the broken hydrogen-bond at the hydrogen side is used to analyze the structure of bulk water, leading to the finding that most molecules are arranged in two hydrogen-bond configurations, in contrast to the picture provided by molecular dynamics simulations. At the liquid-vapor interface, an interplay of surface sensitive measurements and theoretical calculations enables us to distinguish a new interfacial species in equilibrium with the gas. In a similar approach the cluster form of the excess proton in highly concentrated acid solutions and the different coordination of methanol at the vacuum interface and in the bulk can also be clearly identified. Finally the ability of core-level spectroscopies, aided by sophisticated density functional theory calculations, to directly probe the valence electronic structure of a system is used to observe the nature of the interaction between water molecules and solvated ions in solution. Water around transition metal ions is found to interact with the solute via orbital mixing with the metal d-orbitals. The hydrogen-bond between water molecules is explained in terms of electrostatic interactions enhanced by charge rehybridization in which charge transfer between connecting molecules is shown to be fundamental.

The structure, bonding and chemistry of water and hydroxyl on metal surfaces are presented. Synchrotron based x-ray photoelectron- and x-ray absorption spectroscopy along with density functional theory calculations mainly form the basis of the results. Conditions span the temperature range 35 - 520 K and pressures from ultra-high vacuum (~10 fAtm) to near ambient pressures (~1 mAtm). The results provide, e.g, new insights on the importance of hydrogen bonding for surface chemical kinetics. Water adsorbs intact on the Pt(111), Ru(001) and Cu(110) surfaces at low temperatures forming 2-dimensional wetting layers where bonding to the metal (M) mainly occurs via H2O-M and M-HOH bonds. Observed isotope differences in structure and kinetics for H2O and D2O adsorption on Ru(001) are due to qualitatively different surface chemistries. D2O desorbs intact but H2O dissociates in kinetic competition with desorption similar to the D2O/Cu(110) system. The intact water layers are very sensitive to x-ray and electron induced damage. The mixed H2O:OH phase on Ru(001) consists of stripe-like structures 4 to 6 Ru lattice parameters wide where OH decorates the edges of the stripes. On Pt(111), two different long-range ordered mixed H2O:OH structures are found to be inter-related by geometric distortions originating from the asymmetric H-bond donor-acceptor properties of OH towards H2O. Water adsorption on Cu(110) was studied at near ambient conditions and compared to Cu(111). Whereas Cu(111) remains clean, Cu(110) holds significant amounts of water in a mixed H2O:OH layer. The difference is explained by the differing activation barriers for water dissociation, leading to the presence of OH groups on Cu(110) which lowers the desorption kinetics of water by orders of magnitude due to the formation of strong H2O-OH bonds. By lowering the activation barrier for water dissociation on Cu(111) by pre-adsorbing atomic O, generating adsorbed OH, similar results to those on Cu(110) are obtained.

No description available.

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.

The present thesis investigates the electronic and structural properties of adenine, cytosine, and guanine layers on hydrogen passivated silicon (111)(7x7). The (7x7) reconstruction of the silicon surface was achieved by direct current heating of the samples in UHV conditions. After in situ hydrogen passivation layers of the DNA bases were prepared in different thicknesses by means of organic molecular beam deposition, all samples were characterized employing valence band and core level photoemission spectroscopy. Additionally the near edge x-ray absorption fine structure of the DNA base layers was investigated. A detailed and consistent picture of structural and electronic properties of the nucleotide bases in the solid state could be developed by comparison of measurements and DFT/B3LYP calculations.