Amplitude reduction in EXAFS.

Abstract

In real systems, inelastic processes remove photoelectrons from the elastic scattering channel. This reduces the amplitude of the EXAFS causing disagreement between the experimental and theoretically predicted amplitudes. Traditionally these discrepancies were treated by including two semi empirical reduction factors in the data analysis: a mean tree path term, which models the so called extrinsic loss processes, and a constant amplitude reduction factor which accounts for many electron excitations at the absorbing atom. The extrinsic inelastic effects may, however, be modelled more rigorously using a complex exchange and correlation potential. For example the Hedin-Lundqvist (H-L) potential used in most EXAFS data analysis programs. We present a method by which the losses caused by such a potential may be evaluated quickly and easily in the first Born approximation. The losses produced by the H-L potential significantly overestimate those produced by the mean free path alone. Instead the losses appear to agree well with the total reduction given by the semi-empirical reduction factors. These losses do not exhibit the correct low or high energy behaviour but do show excellent agreement with experiment over the range of a typical EXAFS spectrum. We therefore conclude, that the semi-empirical reduction parameters should not be included when data fitting using the H-L potential.