Polarized x-ray-absorption spectroscopy of the uranyl ion: Comparison of experiment and theory.

Abstract

The x-ray linear dichroism of the uranyl ion (${\mathrm{UO}}_{2}^{2+}$) in uranium ${\mathit{L}}_{3}$-edge extended x-ray-absorption fine structure (EXAFS), and ${\mathit{L}}_{1}$- and ${\mathit{L}}_{3}$-edge x-ray-absorption near-edge structure (XANES), has been investigated both by experiment and theory. A striking polarization dependence is observed in the experimental XANES and EXAFS for an oriented single crystal of uranyl acetate dihydrate [${\mathrm{UO}}_{2}$(${\mathrm{CH}}_{3}$${\mathrm{CO}}_{2}$${)}_{2}$\ensuremath{\cdot}${2\mathrm{H}}_{2}$O], with the x-ray polarization vector aligned either parallel or perpendicular to the bond axis of the linear uranyl cation (O-U-O). Single-crystal results are compared to experimental spectra for a polycrystalline uranyl acetate sample and to calculations using the ab initio multiple-scattering (MS) code FEFF 6. Theoretical XANES spectra for uranyl fluoride (${\mathrm{UO}}_{2}$${\mathrm{F}}_{2}$) reproduce all the features of the measured uranyl acetate spectra. By identifying scattering paths which contribute to individual features in the calculated spectrum, a detailed understanding of the ${\mathit{L}}_{1}$-edge XANES is obtained. MS paths within the uranyl cation have a notable influence upon the XANES. The measured ${\mathit{L}}_{3}$-edge EXAFS is also influenced by MS, especially when the x-ray polarization is parallel to the uranyl species. These MS contributions are extracted from the total EXAFS and compared to calculations. The best agreement with the isolated MS signal is obtained by using nonoverlapped muffin-tin spheres in the FEFF 6 calculation. This contrasts the ${\mathit{L}}_{1}$-edge XANES calculations, in which overlapping was required for the best agreement with experiment. \textcopyright{} 1996 The American Physical Society.