X-ray absorption and emission from different atoms of the same compound: success and failure of the single-particle picture

Theoretical x-ray absorption and emission spectra of crystalline CoSi2 and NiSi2 have been computed considering the single-particle matrix elements between one-electron states. The effect of the core hole has been considered. The validity of the proposed models of absorption and emission processes, based on single-particle calculations and the density of state information that can be obtained from the experimental data is investigated. The comparison with the experimental results shows a different behavior for different atoms: a single-particle approach, namely, the final-state rule, describes the process occurring on a Si atom rather well, but no one-electron calculation is able to interpret transitions involving a Co or Ni atom.