Single-site and multisite resonant photoemission theory studied by Keldysh Green’s functions

Abstract

Single (on-site) and multisite (multiatom) resonant photoemission (MARPE) processes are systematically studied by use of nonrelativistic Keldysh Green's function theory. We apply skeleton expansion in terms of renormalized one-electron Green's functions. In this theoretical framework we discuss the importance of the radiation field screening and the dynamically polarized part ${W}_{p}$ in the screened Coulomb propagator. The radiation field screening plays a crucial role in observing the MARPE: We obtain expressions of the resonant processes (on site and multiatom) in terms of an x-ray absorption factor whose imaginary part is proportional to the x-ray absorption intensity. If we neglect ${W}_{p}$, the calculated MARPE intensity is much smaller than the observed one. We also point out the importance of the structure factor in the MARPE analyses. Typically highly symmetric atomic arrangement around a photoemitting atom provides us with no MARPE signal, but outermost oxygen atoms give rise to considerably strong MARPE because of symmetry lowering. On the other hand, low symmetric systems like rutile, perovskite, and $\ensuremath{\alpha}$-alumina $({\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ structures can give rise to finite MARPE even in the case of photoemission from inner layers of perfect crystals. The polarization dependence of the MARPE follows the same selection rule as the main photoemission processes, whereas the O $1s$ MARPE from ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ shows a rather complicated rule. These specific features of MARPE provide useful local structural information.