PNNL

Abstract

Details of the first-shell water structure about Ag+ are reported from a co-refinement of the K- and L2-edge multiple scattering signal in the x-ray absorption fine structure (XAFS) spectra. Detailed fits at the Ag K-edge that include the contributions from multiple scattering processes in the hydrated ion structure cannot distinguish between models containing tetrahedral symmetry versus those containing collinear O-Ag-O bonds. However, we show that the multiple scattering oscillations at the L2- edges have distinctly different phase and amplitude functions than at the K-edge. These phase and amplitude functions depend not only on the scattering symmetry of the multiple scattering paths but also on the nature of the final state electronic wavefunction probed by the dipole allowed transition. Hence the multiple scattering portions of K- and L2- edge spectra provide independent measurements of the local symmetry - not a redundant measurement as is commonly believed. On the basis of the enhanced information content obtained by the simultaneous assessment of both the K and L2-edges we report that the hydrated Ag+ structure consists of both 90° and collinear O-Ag-O bonds that is approximately represented as a distorted trigonal bi-pyramidal structure. Finally the K and L2-edge spectra are used to benchmark the hydration structure that is generated from both DFT-based and classical molecular dynamics simulations. Simulated first-shell structures are compared to the experimental structures.