2p3s3p, 2p3p3p, and 2p3s3s resonant Auger spectroscopy from NiO

We have investigated the behavior of the 2p3s3p, 2p3p3p, and 2p3s3s Auger l ines of NiO, a model compound in the class of strongly correlated 3d system s, while varying the photon energy across the Ni L-3 and L-2 absorption edg es. The experimental data are discussed in comparison with a theoretical mo del based on a charge-transfer multiplet approach. When the excitation ener gy is below the L-3 resonance, we observe the 2p3p3p and 2p3s3p peaks at a constant binding energy. This behavior is typical of nonradiative resonant Raman scattering. If the photon energy is increased further, the 2p3p3p and 2p3s3p lines rapidly transform into constant kinetic energy features, show ing a normal Auger behavior. The transition from Raman- to Auger-like behav ior takes place for photon energies lower than the ones corresponding to ex citations of the photoelectron into ligand-hole states. This might indicate the participation of inelastic processes in the recombination of the core hole involving energies much smaller than the NiO gap, or the possible pres ence of nonlocal effects. On the high photon energy side of the L-3 edge, t he constant kinetic energy of the 2p3p3p and 2p3s3p peaks is systematically larger than the one observed for an excitation well above the L-2,L-3 edge s. We attribute this behavior to the intervention of an intermediate state of 2p(5)3d(10) character, which has very little weight but is strongly enha nced at resonance. [S0163-1829(99)06215-3].