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 (O2pe₆3de₆), multiplet splitting, and crystal-field splitting. The structure of Co 3d levels in CoO (3d^7) is broader than that of Ni 3d levels in NiO (3d^8), in agreement with the energy levels of the final hole state estimated from the Tanabe and Sugano diagrams for the d^6 and d^7 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 O2p3d transitions.