Seminars Archive

Rigorous Theory For XPS and XANES Spectra

Abstract
The structure of X-Ray Photoelectron and X-Ray Absorption Near Edge Spectroscopies, XPS and XANES, contains a wealth of information on the chemical state of the element studied and important insight into this is gained by coupling theory with experiment. Analysis of the spectra observed for transition metal and actinide oxides will be discussed based on results obtained with rigorous ab initio wavefunctions. It will be shown that the spectra reflect not only the oxidation state of an element but also its chemical environment and its chemical bonding with this environment. Most theoretical approaches for modeling XPS and XANES are semi-empirical and adjust parameters to fit the experimental data; by contrast, our theory for isolated atoms and for cluster models of oxides avoids any artifacts that might arise from the use of such parameters The topics to be presented will include: (1) The identification of a new atomic many-body effect that corrects a long standing error in the calculation of XPS multiplet splittings and that does not require an empirical scaling of interaction integrals. (2) Demonstrations of the importance for the predicted XPS and XANES spectra of explicitly treating the covalent interactions of the metal cation with its ligands. And, (3) an analysis of the XPS of actinide cations in terms of atomic many-body effects and multiplet splittings that provides an explanation of the underlying reason that these spectra differ from those for transition metals.