Application of core level spectroscopy to adsorbate and coadsorbate systems

Abstract: Different core level spectroscopies combined with quantum chemistry calculations have been applied to a number of adsorbate system. The objective has been to investigate the modification of the valence electronic structure of the adsorbates due to the chemical bond formed with the substrate atoms. X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) provide information about the occupied electronic states of a system; X-ray absorption spectroscopy (XAS) probes the unoccupied electronic states. In particular, XES and XAS enable atom-specific information to be extracted. For oriented systems electronic states of different symmetry can be separated by means of angle-resolved measurements.Studies have been conducted on the simplest amino-acid, glycine (NH2CH2COOH) adsorbed on Cu(110). This molecule is found to adopt an interesting adsorption geometry, bonding to the surface with its two endgroups. Moreover, glycine adsorbs azimuthally oriented on Cu( 110). Thus, by means of angle-depended XES and XAS, a complete atomspecific partition into 2px, 2py, and 2pz orbital components has been extracted for the occupied and unoccupied, electronic states. The results represent an experimental version of the LCAO (linear combination of atomic orbitals) approach for molecular orbital theory. In order to support the discussion of the glycine results, additional investigations have been performed on its constituent building blocks, e.g, formate and ammonia.In order to characterize the adsorption of CO the role of the substrate, adsorption site, and coadsorbates, i.e., hydragen and potassium, have been addressed. Results from CO adsorbed, e.g., on Ni(lOO) do not support the existing adsorption models based on the interaction of only the frontier orbitals of the adsorbate. The adsorbate cannot be treated as a unit, with only weak modifications due to the influence of the substrate, although the CO adsorption energy only is about 10 % of the dissociation energy. The interaction of the CO π system with the substrate gives rise to a net attraction, forming the covalent bond to the metal. In contrast, the Σ interaction gives rise to a net repulsion due to the lack of an unoccupied CO orbital of proper symmetry favorable for hybridization. It is suggested that many of the interesting chemical properties of adsorbed CO are the result of a balance between these two different hybridization channels.