Multielectron excitations at theLedges of barium in aqueous solution

Abstract

An extensive investigation of the x-ray-absorption spectra above the L edges of a ${\mathrm{Ba}}^{2+}$ water solution is presented. Anomalous features are clearly detected in the spectra and have been associated with the creation of 2p4d and 2s4d double-core hole states for the ${\mathit{L}}_{3}$, ${\mathit{L}}_{2}$, and ${\mathit{L}}_{1}$ edges, respectively. A reliable determination of the double-electron edge parameters has been obtained by performing a combined L-edge analysis in the correct framework of the radial distribution function theory. Ba-O and Ba-H radial distribution functions have been calculated by means of molecular-dynamics simulations and have been used as relevant models in the calculation of the extended x-ray-absorption fine-structure (EXAFS) signals. The simultaneous analysis of the three edges allowed an unambiguous characterization of the multielectron transition affecting the ${\mathit{L}}_{1}$ cross section. The experimental values of the energy onsets of the double-electron features are in good agreement with previous theoretical calculations. The influence of multielectron transitions in the EXAFS data analysis is discussed. The neglect of double-electron excitation effects results in systematic errors on the structural parameters and in particular in an underestimation of the coordination numbers. \textcopyright{} 1996 The American Physical Society.