STRUCTURES OF SURFACES AND AMORPHOUS SAMPLES OBTAINED BY EXAFS MEASUREMENTS IN THE X-RAY RAMAN-SCATTERING MODE

We present a theory for extended x-ray-absorption fine structures (EXA FS) measured in the x-ray Raman-scattering mode. Spectra of such struc tures can be obtained by scanning the frequency of the exciting x-ray photons over the EXAFS region while fixing the detector frequency to a n emission resonance. It is shown that these spectra in addition to th e structure-determining features of ordinary EXAFS allow us to obtain orientational information, that is, to obtain data on both bond length s and on orientations of bonds. We derive the explicit dependence of s pectral features on the angles between the bond direction and the dipo le moment direction of an x-ray-emission transition. Since each resona nce frequency is associated with a specific direction of the transitio n dipole, rich possibilities prevail for orientational information wit hin a molecular framework. We show that measurements of extended xray- absorption fine structures in the x-ray Raman-scattering mode also all ow us to obtain bond angles of disordered systems such as glasses or f ree molecules, for which conventional x-ray-diffraction methods do not apply. A comparative discussion is given on the prospects of EXAFS ex periments in the nonradiative (Auger) scattering mode.