Resonant Anger spectroscopy at the OK edge of NiO

We have measured the photon-energy dependence of the O 1s2s2s and O 1s2s2p resonant Auger lines at the O K edge of NiO. The transition from the Raman-like regime, where these features are observed at constant binding energy, to the Auger regime, where they appear at constant kinetic energy, is found for a photon energy similar to 0.8 eV lower than the absorption maximum at threshold, usually assigned to O 2p states hybridized with the Ni 3d orbitals. We interpret this behavior as a consequence of the fact that the intermediate states of the resonant Auger process also have Ni 4sp character. At the photon energy where the Raman- to Auger-like regime transition takes place, the constant kinetic energy of the Auger features is similar to 1.8 eV higher with respect to off-resonance excitation. Analysis of the photon-energy dependence of the kinetic energy of the Auger peaks across the transition from the Raman-like to the Auger-like regime allows us to determine the onset of the Ni 4sp continuum in the O K edge absorption spectrum. [S0163-1829(99)03031-3].