Ab initio calculation of extended x-ray-absorption fine structure in Br2

A quantitative theory of extended x-ray-absorption fine structure (EXAFS) in diatomic molecules is presented and tested by ab initio calculations in Br2. The theory, based on a refinement of conventional EXAFS theory, takes into account (1) an energy-dependent exchange-correlation potential, (2) multielectron excitations, and (3) a single-scattering, spherical-wave expansion. Inelastic processes are included assuming that core-hole excitations and losses in propagation are uncorrelated. We find that a Dirac-Hara exchange potential gives better overall agreement of the EXAFS phase than does the Hedin-Lundqvist potential. The amplitude discrepancy between experiment and single-particle theory can be corrected by adding core-hole lifetime effects, experimental resolution, and multielectron excitations.