X-ray L_2,3 resonant Raman scattering from NiO: Spin flip and intermediate-state relaxation

We present measurements of x-ray resonant Raman scattering from NiO with L_2,3 excitation and calculations in an ionic model based on the process (2p⁶3s²3d⁸)→(2p⁵3s²3d⁹)→(2p⁶3s¹3d⁹). The agreement between theory and experiment is good from below threshold up to 2.5–3 eV above threshold. By comparing theory and experiment, we show the effect of crystal-field splitting and the contribution bringing the Ni²⁺ triplet ground state to a singlet final state with excitation above the L₃ edge. When the excitation energy exceeds the L₃ threshold by about 3 eV, we present evidence that the intermediate state in the scattering can also relax to a state similar to that created with threshold excitation. We suggest d-d excitation and electron-hole pair excitation as the origin of this relaxation.