Hard X-ray resonant electronic spectroscopy in transition metal oxides

K-edge X-ray absorption and 2p-XPS spectra of 3d-element oxides present spectral features which cannot be explained within a simple one-electron model. These features reveal the fine electronic structure of transition metal (TM) oxides valence states resulting from hybridized TM-3d and O-2p states, and the correlations between these valence electrons. In this paper, we show how resonant electronic spectroscopy (resonant Auger or resonant photoelectron spectroscopy) around the TM K-edge can be used to interpret the structures of the threshold and, with the help of theoretical calculation, to determine the electronic configuration of the excited ion. Quadrupolar transitions towards localized 3d orbitals are hence detected and quantitatively characterized in the titanium K-edge prepeaks in TiO2 and in the Ni K-edge prepeaks in NiO by angular-dependent resonant KLL Auger measurements. Valuable information also seems to be available in the resonant behavior of 2p-XPS spectra of NiO and Fe2O3.