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${\mathrm{O}}_6^{10-}$ cluster and the sudden approximation (monopole selection rules). The spectra are shown to exhibit the effects of monopole charge-transfer shake-up ($\mathrm{O}2p{e}_g\to \mathrm{Co}3d{e}_g$), 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 $\mathrm{O}2p\to \mathrm{Co}3d$ transitions.