X-RAY ABSORPTION ON URANIUM SYSTEMS AT VARIOUS THRESHOLDS

Abstract
We have studied the near edge X-ray absorption fine structure of various metallic and non-metallic U compounds at the L₃, M₃, M_4,5, N_4,5, and O_4,5 thresholds using transmission and total-electron-yield techniques. At the L₃ thresholds, a chemical shift of ≈4 eV between trivalent and tetravalent U compounds is observed, which drops to ≈2eV between the latter and hexavalent U systems. This indicates extended 5f character and a 5f count close to two in the high oxidation states. The M_4,5 thresholds are characterised by intense absorption peaks due to 3d-5f transitions which show no multiplet structure from 3d-5f exchange interaction. However, the linewidths of the M_4,5 peaks decrease by ≈20% with increasing U-U distance in metallic systems, indicating a narrowing of the 5f band. The soft X-ray absorption spectra at the N_4,5 thresholds behave very similarly to the M_4,5 edges, except for a vanishing intensity of transitions into the continuum. Rich structures from 5d-5f exchange interaction are observed at the O_4,5 edges of non-metallic U compounds, becoming broadened and partly vanishing in metallic systems, presumably due to the more itinerant nature of the 5f states.