PNNL

Abstract

Ab initio, fully relativistic four component theory was used to determine atomic many-body effects for the 4f X-ray photoelectron spectra, XPS, of U5+ and U4+ cations. Many-body effects were included through the use of configuration interaction, CI, wavefunctions, WF’s, that allow the mixing of XPS allowed and XPS forbidden configurations. This work extends our earlier study of the U 4f XPS in that the orbitals for the final, ionic states of the cations are allowed to relax in the presence of the 4f core-hole. In the earlier work, orbitals optimized for the initial state were frozen and also used for the final, ionic states. While the main XPS features are similar in both cases, using relaxed orbitals for the ionic states introduces changes in the widths of the peaks and in the 4f5/2 and 4f7/2 spin-orbit splitting. The major effect for the U5+ XPS is a broadening of these peaks, which is ascribed to the contraction of the 5f orbitals for the ionic configuration. For the U4+ XPS, increases of the energy separations are also present but are offset by a decrease in the artificial configuration mixing due to the use of non-optimal orbitals for the 5f shell in the frozen orbital approximation. The extent of configuration mixing for the U5+ and U4+ final state WF’s is characterized by the magnitude of the intensity stolen by the XPS forbidden configurations. Overall the use of relaxed orbitals improves the agreement between the theoretical XPS for the U4+ cation and the experimental measurements for UO2