Multielectron excitations at the L edges of barium in aqueous solution

An extensive investigation of the x-ray-absorption spectra above the L edges of a Ba2+ 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 L(3), L(2), and 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 L, 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.