Intrinsic and Extrinsic Losses in XAFS

Abstract
Interference between extrinsic and intrinsic losses in x-ray absorption spectroscopy is treated within a Green's function formalism, without explicit reference to final states. The approach uses the a quasi-boson representation [W. Bardyszewski and L. Hedin, Phys. Scripta 32, 439 (1985); J. de Physique C 9 . 1101 (1987)] and perturbation theory to second order in the interaction potential. We find that extrinsic and intrinsic losses cancel near excitation thresholds while the strength in the primary channel is increased. Above threshold the theory rapidly crosses over to the sudden-approximation limit. Calculations are presented which illustrate the theory and its implications for XAFS analysis. The results show that satellite terms are generally weak and that an asymmetric Fano Une shape is needed to describe the quasi particle peak of the spectral function.