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' 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.