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

Abstract

The x-ray-photoelectron valence-band and inner-shell spectra of CoO are presented and interpreted using the molecular-orbital theory for the Co${\mathrm{O}}_{6}^{10\ensuremath{-}}$ cluster and the sudden approximation (monopole selection rules). The spectra are shown to exhibit the effects of monopole charge-transfer shake-up ($\mathrm{O}2p{e}_{g}\ensuremath{\rightarrow}\mathrm{Co}3d{e}_{g}$), multiplet splitting, and crystal-field splitting. The structure of Co $3d$ levels in CoO ($3{d}^{7}$) is broader than that of Ni $3d$ levels in NiO ($3{d}^{8}$), 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 $\mathrm{O}2p\ensuremath{\rightarrow}\mathrm{Co}3d$ transitions.