Theoretical study of the crystal field excitations in CoO

A theoretical investigation of the crystal field excitations in CoO is presented. Special attention is given to the excitation energy of the 4A2g state. In recent experimental and theoretical studies an excitation energy around 3.1 eV was reported. This is in disagreement with the 2.1 eV deduced from optical spectroscopy data. After analyzing electron correlation effects, spin–orbit interactions and the material model to represent the CoO crystal, we can confirm the interpretation of the optical data, not only for the 4A2g state but also for all other low-lying crystal field excitations. Electron correlation effects are found to have a significant differential effect on the excitation energies, ranging from +0.3 to –0.6 eV. Spin–orbit interactions are less important, affecting the excitation energies by at most 0.05 eV. Finally, we discuss the effect of the Pauli repulsion between the cluster ions and the first shell of ions around the cluster. This affects the excitation energies by a small, but significant, amount.