XAFS theory studied by closed-path Green's function

The basic framework of X-ray absorption fine structure (XAFS) theory is formulated by use of the non-equilibrium Keldysh-Green function, which presents a unified view of XAFS spectra at zero and nonzero temperature. In particular, the relation between the scattering Green's function and the retarded Green's function G^r is discussed, along with resonance effects in XAFS. The latter effects give rise to unexpected peaks within one-electron theory. Loss effects, both intrinsic and extrinsic, are also discussed.