Structures of surfaces and amorphous samples obtained by EXAFS measurements in the x-ray Raman-scattering mode.

Abstract

We present a theory for extended x-ray-absorption fine structures (EXAFS) measured in the x-ray Raman-scattering mode. Spectra of such structures can be obtained by scanning the frequency of the exciting x-ray photons over the EXAFS region while fixing the detector frequency to an emission resonance. It is shown that these spectra in addition to the structure-determining features of ordinary EXAFS allow us to obtain orientational information, that is, to obtain data on both bond lengths and on orientations of bonds. We derive the explicit dependence of spectral features on the angles between the bond direction and the dipole moment direction of an x-ray-emission transition. Since each resonance frequency is associated with a specific direction of the transition dipole, rich possibilities prevail for orientational information within a molecular framework. We show that measurements of extended x-ray-absorption fine structures in the x-ray Raman-scattering mode also allow us to obtain bond angles of disordered systems such as glasses or free molecules, for which conventional x-ray-diffraction methods do not apply. A comparative discussion is given on the prospects of EXAFS experiments in the nonradiative (Auger) scattering mode.