THEORY OF MANY-BODY EFFECTS IN VALENCE, CORE-LEVEL AND ISOCHROMAT SPECTROSCOPIES ALONG THE 3D TRANSITION-METAL SERIES OF OXIDES

Core level X-ray photoemission and absorption spectra (XPS and XAS) are calculated for some transition metal (TM) insulating compounds, especially oxides, using a configuration-interaction impurity-Anderson model combined with the influences of the core effect. Our model calculation is able to explain well the core-XPS of a series of early TM compounds, characterized by a nominally 3d(0) configuration in the ground state, including those compounds which involve pre-transitional elements such as potassium or calcium. The standard ''linear muffin-tin orbital'' (LMTO) band structure for TiO2 compounds in rutile structure is recalled and compared to experimental valence photoemission spectra. A few characteristics of the outcoming density of states (DOS) are used to interpret the Ti2p XPS in the simplest way within the previously considered Anderson model, however, for the onset of the Ti K-edge XAS, the calculated DOS of TiO2 does not predict the very first weak peak, indicating that this feature is not simply due to a one-electron (dipole) transition and needs to be interpreted by taking into account both dipole and quadrupole transitions; moreover it is necessary to take into account an additional phenomenon, i.e. the Is core hole effect, which is characteristic of the Ti K-edge and of course absent from the Bremsstrahlung isochromat spectroscopy (BIS) as is shown from a comparative study between XAS and BIS. For the interpretation of the core level spectra, the considered early TM oxides (TiO2 for example) resemble the late TM oxides, such as MnO, ..., CuO, and belong to the charge transfer regime; in the case of CuO, in order to explain the 2p XPS satisfactorily, the full multiplet coupling between a Cu3d hole and a Cu2p core hole is taken into account together with the corresponding spin-orbital interactions at the expense of considering only the cluster version of the Anderson model.