Many-body aspects of the near-edge structure in x-ray absorption

The near-edge structure (NES) in x-ray absorption is investigated using a simple reformulation of the Hartree-Fock approach. It is found that the most natural single-particle states are not the usual Hartree-Fock ones but those that explicitly incorporate the exclusion principle in the transition by having the directly excited photoelectron state orthogonal to all initially occupied states and the resulting core hole orthogonal to all finally occupied states. This reformulation shows clearly that many-body effects are important when the perturbation by the core-hole potential is significant. Only in two limits does the NES reduce to an effective single-particle problem, namely, when a single photoelectron either (a) makes a transition into an initially empty shell (band) or (b) fills that shell (band). In the former case the local density of states of the final potential is appropriate, while for the latter the density of states of the initial system, before excitation, is appropriate. Spin-dependent effects are illustrated by a model calculation.