Double-electron excitation channels at the Ca2+K-edge of hydrated calcium ion

To investigate the effect of multielectron transitions on the EXAFS data analysis at the Ca K-edge, we collected spectra of Ca2+ in aqueous solution. The anomalous peaks detected in the experimental data have been assigned to the simultaneous excitations of the [1s3p] and [1s3s] electrons. An accurate determination of the hydration geometry of the Ca2+ ion has been performed. The hydrogen atoms have also been included in the EXAFS data analysis, and a Ca−H distance of 3.10 Å has been obtained. The influence of multielectron excitations on the quantitative extraction of the structural parameters from the EXAFS spectrum has been investigated. Omission of double-electron excitation edges from the atomic background significantly worsens the quality of the EXAFS fits and results in unreliable values of the Ca−O coordination number and Debye−Waller factor. A proper determination of the coordination geometry of calcium compounds can be obtained only if multiexcitations effects are accounted for.