THEORETICAL EVIDENCE FOR THE EXISTENCE OF EXCITONS IN MGO

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