  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.
