The effect of symmetry on the U L3NEXAFS of octahedral coordinated uranium(vi).
We describe a detailed theoretical analysis of how distortions from ideal cubic or Ohsymmetry to tetrahedral, D4h, 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 Ohsymmetry. 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 Ohderived egorbitals. 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.
Department of Chemistry, University of North Texas, Denton, Texas 76203-5017, USA.
Paul S
0000000349315217
PS
Bagus
Consultant, Austin, Texas 78730, USA.
Connie J
CJ
Nelin
Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
Eugene S
ES
Ilton