Computational studies of core level XPS satellites in transition metal systems : Department of Quantum Chemistry, Uppsala University

Abstract: Upon core ionization of a free molecule or a molecule adsorbed on a metal surface, higher binding energy satellites are observed, so called "shake-up" satellites, which can be viewed as valence excitations simultaneously to the creation of the core hole. In this thesis, theoretical studies of such core level XPS satellites in transition metal systems are presented, as well as cluster calculations of XES spectra, using the quantum mechanical INDO method. The main motivation of these studies is to get a more detailed understanding of the bonding between the adsorbed molecules and the metal and the screening process accompanying the creation of the core hole. This interest is motivated by fundamental questions about the adsorbate-substrate interaction as well as technical applications, such as catalysis.It has been found that many aspects of extended adsorbate systems, where CO is coordinated to metal atoms, can be modelled by transition metal carbonyls such as Mo(CO)6, Cr(CO)6 and Ni(CO)4. The bonding between the core ionized CO group and the transition metal, consists of an interaction between, on the one hand, the sigma lone pair of the ligand and, on the other hand, the metal d-orbitals and the CO antibonding pi-star orbital of the CO-group, i.e. a sigma donation from the ligand and pi donation from the metal. It is the changes in this bonding which are responsible for the dominant features of the shake-up spectra in the studied model molecules. The main conclusion of the three abovementioned case studies is that new excitations involving charge transfer from the metal to the ionized ligand occur with significant probability in the metal carbonyls, in addition to the internal excitations present also in free CO.In extending the model towards larger systems, calculations of CO/Ni(100), N2//Ni(100), CO/Cu(100) and benzene/Cu(100) were undertaken, where the metal surface was modelled by clusters of 1-12 metal atoms. Many of the features from the metal carbonyls are found also in these systems. The most important added feature is the presence of high-intensity metal-metal excitations, which mainly affect the shape of the main line.Ground state cluster calculations were performed to explain the XES spectra of CO adsorbed on Ni(100) and Cu(100) surfaces. It was found that the calculations reproduce the important characteristics of the experimental spectra, and were used as a support for the assignments of the involved states.