Dynamic screening of a core hole. II. Application to EXAFS

Abstract

For pt.I see ibid., vol.9, p.1189 (1979). The dynamic screening model describing the relaxation of the conduction electrons after the absorption of an X-ray photon in a simple metal is extended to the physical situation of EXAFS. It is used to calculate the potential of the central atom as a function of the velocity of the photoelectron and to derive the corresponding phaseshifts delta l. They show a transition between a neutral-like behaviour at low energy and an ionic-like one at high energy. This transition takes place in the whole energy range (above about 1 keV) and not only around the threshold for plasmon emission. The authors deduce from these results the accuracy of the determination of distances that can be expected when using ionic phase-shifts in a metal. This precision is found to depend on two parameters: (i) the distance of the neighbouring atom; and (ii) the energy Emax of the higher part of the EXAFS spectrum. The problem of the transferability of phaseshifts is thus discussed in a more quantitative way. In addition, they obtain the reduction of the amplitude of the EXAFS oscillations due to plasmon losses; this amplitude is greater than the one which is predicted by the mean free path factor e-2R lambda /, because of the transient processes of the screening mechanism.