Theoretical evidence for the existence of excitons in MgO

The ab initio Hartree-Fock cluster model method has been used to investigate the existence of excitons in bulk MgO. Excitations either from the oxygen 2p valence band orbitals or from the oxygen 1s core orbitals to unoccupied cluster orbitals always lead to delocalized orbitals representative of the MgO conduction band. However, when the electron is excited from a 1s or 2p core level of Mg, the occupied level is found to be localized around the atom with the core hole. These results, for the first time, give strong direct evidence for excitonic states in oxide crystals.