GENERAL MULTIPLE-SCATTERING SCHEME FOR THE COMPUTATION AND INTERPRETATION OF X-RAY-ABSORPTION FINE-STRUCTURE IN ATOMIC CLUSTERS WITH APPLICATIONS TO SF6, GECL4, AND BR2 MOLECULES

We present a general multiple-scattering (MS) scheme utilizing complex potentials for the calculation and interpretation of inner-shell x-ray-absorption fine structure for clusters, of atoms in condensed matter with application to the case of the molecules SF6, GeCl4, and Br2. The method is based on the solution of the Dyson equation with a complex self-energy of the Hedin-Lundqvist type for the single-particle Green's function describing the propagation of the excited photoelectron through the system. We also address the problem of the equivalence between two traditional approaches to this kind of calculation: the scattering approach of Dehmer and Dill and the density-of-states (or MS) approach of Lee and Pendry. The application of the theory to the calculation of the extended fine structure in the three molecules shows that one can get amplitude agreement without introducing intrinsic loss reduction effects, which seem therefore to be negligible. Some minor problems with absolute phase agreement still remain in some cases.