Interference between Extrinsic and Intrinsic Losses in XAFS

The interference between extrinsic and intrinsic losses in x-ray absorption fine structure (XAFS) is treated within a Green's function formalism, without explicit reference to final states. The approach makes use of a quasi-boson representation of excitations and perturbation theory in the interaction potential between electrons and quasi-bosons. These losses lead to an asymmetric broadening of the main quasi-particle peak plus an energy-dependent satellite in the spectral function. The x-ray absorption spectra (XAS) is then given by a convolution of an effective spectral function over a one-electron cross-section. It is shown that extrinsic and intrinsic losses tend to cancel near excitation thresholds, and correspondingly, the strength in the main peak increases. At high energies, the theory crosses-over to the sudden approximation. These results thus explain the observed weakness of multi-electron excitations in XAS. The approach is applied to estimate the many-body corrections to XAFS, beyond the usual mean-free-path, using a phasor summation over the spectral function. The asymmetry of the spectral function gives rise to an additional many-body phase shift in the XAFS formula.