High-energy spectroscopic study of the occupied and empty electronic levels in uranium metal and compounds

Abstract

X-ray photoemission spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS) have been used to investigate the occupied and empty density of states (DOS) of α-uranium and of the compounds UO 2, US, and UAs. At the photon energy (1486.6 eV) used in both methods, the l = 3 projected DOS is bringing the dominant contribution to the experimental spectra. In the pure metal, the total 5f DOS appears as a 4 eV wide band showing a splitting. A completely different situation is encountered in the insulator UO 2 which contains two localized 5f electrons. Discrete XPS (5f 1) and BIS (5f 3) final states are observed on each side of E F, allowing the first determination of the Coulomb correlation energy for 5f electrons. In the occupied part of the band the p states are clearly separated from the 5f peak. The XPS spectra of US and UAs show similarly separated p and f structures but the metallic character of these compounds is recognized by the high DOS at E F. Their BIS spectra are formed by a dominant peak followed by an intense tail extending far above E F. This feature is attributed to strong many-body effects, as demonstrated by the line shape of the U core levels in these compounds. Satellites involving discrete excitations to empty 5f states are observed in the spectra of UO 2 and Th metal.