Effect of multielectronic configurations on the XAFS analysis at the FeK edge

An extensive investigation of the extended X-ray absorption fine structure (EXAFS) spectra of Fe2+ and Fe3+ in aqueous solution is presented. Anomalous peaks at 3.9 and 5.3 Angstrom(-1) have been detected and assigned to the simultaneous excitations of the 1s3p and 1s3s electrons, referred to as KM23 and KM1 channels. The Fe3+ water solution absorption cross section displays the presence of peculiar features that reflect the existence of two electronic configurations in the ground state, giving rise to two channels of comparable intensity. The influence of many-body effects on the quantitative extraction of the structural parameters from the EXAFS spectra was investigated. Omission of double-electron excitation edges from the atomic background significantly worsens the quality of the EXAFS fits and results in slightly incorrect values of the structural parameters. Conversely, the [Fe(H2O)(6)](3+) structural parameters obtained from the EXAFS data analysis, performed in the framework of the one-electron approximation, are affected by severe systematic errors, despite the perfect agreement between the experimental and theoretical data. An accurate determination of the hydration geometry of the Fe3+ ion can be only obtained using a theoretical scheme based on the multichannel multiple-scattering theory. The hydrogen contribution has been included in the EXAFS data analyses and Fe-H distances of 2.84 and 2.76 Angstrom have been obtained for Fe2+ and Fe3+, respectively, which are in excellent agreement with the results of ab initio quantum-mechanical/molecular-mechanical molecular dynamics simulations.