Many-body effects in the 4f x-ray photoelectron spectroscopy of the U⁵⁺ and U⁴⁺ free ions

A strictly ab initio many-electron theory was used to calculate the 4f x-ray photoelectron spectroscopy (XPS) of the free U⁵⁺ and U⁴⁺ ions. The calculations, based on relativistic Dirac-Fock self-consistent field (DF-SCF) and Dirac configuration interaction (DCI) wave functions (WF’s), indicate that the atomic spectra have a considerable multiplet structure. However, the multiplet splitting, which is mainly manifest as a broadening of the 4f_5∕2 and 4f_7∕2 lines, is not as strong as for the first-row transition metals. As expected, the U⁴⁺ primary peaks are broader and have more associated satellite structure than does U⁵⁺. A comparison of a synthetic spectrum for U⁴⁺ with the observed XPS of UO₂ indicates that interatomic, solid-state, effects may decrease the multiplet and spin-orbital splitting, relative to the free ion. Notably, the 7 eV satellite characteristic of UO₂ is absent from the calculated XPS of U⁴⁺.