The effect of symmetry on the U L₃ NEXAFS of octahedral coordinated uranium(vi)

We describe a detailed theoretical analysis of how distortions from ideal cubic or O_h symmetry to tetrahedral, D_4h, symmetry affect the shape, in particular the width, of the U L₃-edge NEXAFS for U(vi) in octahedral coordination. The full-width-half-maximum (FWHM) of the L₃-edge white line decreases with increasing distortion from O_h symmetry. In particular, the FWHM of the white line narrows whether the tetragonal distortion is to compression or to extension. The origin of this decrease in the FWHM is analyzed in terms of the electronic structure of the excited levels arising from the unoccupied U(6d). The relative importance of ligand field and of spin-orbit effects is examined, where the dominant role of ligand field effects is established. Especially at higher distortions, the ligand splittings decrease rapidly and lead to an accelerated, quadratic decrease in the FWHM with increasing distortion. This is related to the increase of covalent character in the appropriate component of the O_h derived e_g orbitals. Our ab initio theory uses relativistic wavefunctions for cluster models of the structures; empirical or semi-empirical parameters were not used to adjust prediction to experiment. A major advantage is that it provides a transparent approach for determining how the character and extent of the covalent mixing of the relevant U and O orbitals affect the U L₃-edge white line.