Extended x-ray-absorption fine-structure amplitudes—Wave-function relaxation and chemical effects

Experimentally measured amplitudes of the extended x-ray absorption fine structure (EXAFS) are smaller, typically by a factor of ½ than results calculated from one-electron theories. We suggest that this reduction factor may be largely accounted for by a theory based on the relaxation of passive electron orbitals accompanying photoemission, which permits a straightforward approximate calculation in terms of a many-body overlap integral. Using ab initio Hartree-Fock-Roothaan calculations and estimates for correlation effects, the theory gives reduction factors of 0.60, 0.64, and 0.64±0.04 for ${\mathrm{F}}_2$, ${\mathrm{Cl}}_2$, and ${\mathrm{Br}}_2$, respectively, in rough agreement with experiment. These numbers are 7%-14% smaller than those for free halogen atoms, and indicate that chemical effects (in particular, molecular charge transfer) are important in determining the EXAFS reduction factor. This charge transfer also affects the overall EXAFS phase. The reduction in single-electron excitation is made up by multielectron excitations, the "shake-up," and "shake-off" processes. The possible contributions to EXAFS from these channels are discussed and estimated to be about 10% of the total amplitude.