Theoretical Studies on Many-Body Effects in Core Spectroscopies by Nonequilibrium Green 's Functions

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Many-body theory of X-ray photoelectron diffraction(XPD) spectra is developed by using Keldysh's nonequilibrium Green's functions, which was basically applied by Caroli et al. We have applied skeleton diagram expansion in terms of dressed one-electron Green's function G and the screened Coulomb interaction W. The damping photoelectron wave functions under the influence of the optical potential play a central role of the present XPD theory. Based on the present theoretical framework, we give detailed discussion on Debye-Waller and Franck-Condon factors in XPD spectra at finite temperature, resonance effects in particular multi atom resonant photoemission(MARPE) effects, and also on the loss effects. The XPD spectra is simply described by a product of the Franck-Condon factor and the XPD intensity including the Debye-Waller factor. This result is contrast to the XAFS where the Franck-Condon factor is smeared out by a convolution product. We have discussed the conditions which give rise to prominent MARPE spectra. In particular symmetry lowering plays an important role to give rise to prominent MARPE spectra. The angular dependence provides us useful information on near neighbor atomic arrangement. Some model calculations clarify the basic physical processes of MARPE spectra. To discuss the loss spectra, we stress the importance of the interference effects between the intrinsic and extrinsic losses. The explicit formulas which include both inelastic and elastic scatterings are newly derived in the present theoretical framework. These formulas enable us to calculate XPD pattern associated with losses. We also have found the importance of the radiation field screening to describe the resonance effects in particular MARPE. As we realized the quantum electrodynamic(QED) theory, the project to include optical field effects in XPS and X-ray absorption spectra.