X-ray-photoelectron spectroscopic studies of the electronic structure of CoO

The x-ray-photoelectron valence-band and inner-shell spectra of CoO are presented and interpreted using the molecular-orbital theory for the CoO₆^10- cluster and the sudden approximation (monopole selection rules). The spectra are shown to exhibit the effects of monopole charge-transfer shake-up (O2peg→Co3deg), multiplet splitting, and crystal-field splitting. The structure of Co $3d$ levels in CoO (3d⁷) is broader than that of Ni $3d$ levels in NiO (3d⁸), in agreement with the energy levels of the final hole state estimated from the Tanabe and Sugano diagrams for the d⁶ and d⁷ configurations. Shake-up satellite structures lying 5 to 10 eV below the main peaks in the Co inner-shell and valence-band levels suggest that the CoO optical-reflectance structure at 5.5 and 7.5 eV is associated with O2p→Co3d transitions.