AB-INITIO STUDY OF LOCAL D-D EXCITATIONS IN BULK COO, AT THE COO(100)SURFACE, AND IN OCTAHEDRAL CO2+ COMPLEXES

Quantum chemical ab initio calculations are presented for the lowest d -d excitation energies of Co2+ ions in bulk CoO, at the CoO(100) surfa ce and, for comparison, in CoF2, in [Co(NH3)(6)](2+) complexes, and fo r Co2+ impurity ions in LiF. Different cluster models have been used f or describing the octahedral or distorted octahedral surrounding of th e Co2+ ions: A pure point-charge model, a CoO610- cluster (CoO58- at t he surface) embedded in a point-charge (Madelung) field, and a CoO610- cluster surrounded by 18 effective core potentials in the next positi ve coordination shell and embedded in the point-charge held. The calcu lations for the ground state and the lowest excited states have been p erformed at the complete active space self-consistent field, valence c onfiguration interaction, and multiconfiguration coupled-electron pair approach levels. The best results for the lowest excitation energies for bulk CoO are 0.80 eV (T-4(2g)) and 1.71 eV ((4)A(2g)) which is sli ghtly lower than experimental data derived from optical and electron-e nergy-loss (EEL) spectra. At the CoO(100) surface, the threefold spati al degenerate T-4(1g) ground state of bulk CoO is split by about 60 me V into a lower (4)A(2) and a higher (4)E component. The same small spl itting has been observed experimentally in high-resolution EEL spectra as a low-energy shoulder of the first Fuchs-Kliewer surface phonon. T he splitting of the first excited T-4(2g) state of CoO at the CoO(100) surface is similar to 0.4 eV and gives rise to a surface state at 0.3 5 eV (experimentally at 0.45 eV).