Rigorous theoretical studies of the electronic structure to describe the near edge X-ray adsorption fine structure,NEXAFS, of different oxidation states of U in UOx are reported. The primary focus is for the U L3-edge NEXAFS where the origin of differences in the observed spectra as a function of the oxidation state is established; however, aspects of the O 1s-edge will also be considered. Key features of the L3-edge NEXAFS are related to the ligand field splitting of the excited state orbitals. In particular, the ligand filed splitting is related to the different extents of covalent mixing that occur at different U-O distances for the different oxidation states. The theoretical relative energies and intensities are based on the values obtained with electronic wavefunctions for cluster models of the oxides, which provide a way to establish the relationship between the covalent mixing and key features of the NEXAFS spectra. In addition, correlations are established between the width of the L3-edge NEXAFS and the U-O distance.

Theoretical analysis of the NEXAFS in U oxides

247th National Meeting (2014)

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