Electronic surface states of CoO(100): an electron energy loss study

We identify electronic excitations in the first layer of a CoO(100) surface applying electron energy loss spectroscopy. These excitations are localized within the optical band gap of CoO and correspond to excitations within the d orbital manifold of the Co2+ ions. The excitations of Co ions in the surface are different from those of Co ions octahedrally (sixfold) coordinated as in the bulk, because the ligand field at the surface is reduced due to the lower coordination (fivefold) at the surface. Two surface states can be identified in conjunction with ab initio calculations, one at 50 meV, i.e. below the Fuchs-Kliewer surface phonon, and another one at 0.45 eV below the first excited state of sixfold coordinated Co ions. The surface states are quenched upon gas adsorption.