Resonant Inelastic X-Ray Scattering at the OxygenKResonance of NiO: Nonlocal Charge Transfer and Double-Singlet Excitations

We report high-resolution polarization-dependent resonant inelastic x-ray scattering (RIXS) at the O $K$ resonance of NiO showing a rich excitation spectrum. We perform multisite ${\mathrm{Ni}}_6{\mathrm{O}}_{19}$ cluster model calculations, revealing that solid state effects are substantial. We identify a nonlocal charge transfer excitation at 4–5 eV and double-singlet creation at 1.75 eV, both exhibiting significant scattering geometry dependence. Apart from an intense band of local charge transfer excitations (above 5 eV) also $dd$ excitations at 1 eV are observed. Finally, we point out that O $K$ RIXS of correlated metal oxides allows a quantitative and consistent determination of the charge transfer energy $\Delta$ and the Hund coupling energy $J_H$.