Probing molecular adsorbates with core level spectroscopies : Electronic structure and bonding models

Abstract: Resonantly excited X-ray emission spectroscopy has been applied to study the valence electronic structure of molecular adsorbates in an atom specific and orbital symmetry selective manner. In combination with ab initio cluster calculations, electronic structure and bonding models have been derived. Existing models of surface chemical bonding have been reviewed and partially revised.Most notably, the bonding mechanism of carbon monoxide (CO) on transition and noble metals has been revised and is found to be the result of a strong covalent interaction between the CO orbitals and the metal bands within each orbital symmetry. A characteristic allylic configuration is found in the π-system and strong polarization within the (Σ system. The equilibrium properties of adsorbed CO are the direct result of a balance between the repulsive Σ-interaction and the attractive π-interaction; both in terms of the total energy and the local bond properties. The bonding of ammonia (NH3) on the Cu(ll0) surface is found to be dominated by a large covalent interaction, which contrasts the previous model of a strong electrostatic interaction. Furthermore, adsorbate-adsorbate interaction leads to a tilted adsorption geometry. Ethylene (C2H4) on Cu(ll0) is adsorbed in the di-Σ configuration, according to the generally accepted Dewar Chatt Duncanson model for hydrocarbon adsorption. The application and interpretation of resonantly excited X-ray emission on these systems also required a thorough discussion of the spectroscopic process.Another topic was the vibrational fine structure in the X-ray photoemission corelevel main lines of adsorbed molecules. The observation of the vibrational fine structure in molecular adsorbates is remarkable, as it was previously thought impossible to observe due to solid state broadening contributions. A detailed analysis of the vibrational fine structure and the line profile makes it possible to study the electronic and geometric properties of the core-ionized adsorbate and the dynamic response to core-ionization in unprecedented detail. A connection between the vibrational fine structure and the local chemical environment has been found. Furthermore, our basic understanding of the photoemission process on adsorbates is put to the test in the photon energy dependence of the vibrational fine structure.