Multielectron configurations in the x-ray-absorption near-edge structure of NiO at the oxygen K threshold

The oxygen I] -edge x-ray-absorption near-edge structure of stoichiometric NiO has been measured, using partial electron yield, at the "grasshopper" line of the Frascati synchrotron-radiation facility. The main features of the spectrum in the multiple-scattering region over the 50-eV energy range beyond threshold are well predicted by the full multiple-scattering one-electron theory. The interpretation of the fine details of the spectrum requires a many-body picture; in fact, the results are in agreement with a description of NiO as an intermediate valence system, where the charge-transfer gap is smaller than Hubbard correlation.