Hydrated structure of Ag(I) ion from symmetry-dependent, K- and L-Edge XAFS multiple scattering and molecular dynamics simulations

Details of the first-shell water structure about Ag⁺ are reported from a corefinement of the K- and L₂-edge multiple scattering signal in the X-ray absorption fine structure (XAFS) spectra. Detailed fits of the Ag K-edge data 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 L ₂-edges have distinctly different phase and amplitude functions than at the K-edge. These phase and amplitude functions depend not only on the symmetry of the multiple scattering paths but also on the nature of the final state electronic wave function 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 contains five or six water molecules in the first shell with a significant number of nearly collinear and 90° O-Ag-O bond angles. 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.