ELECTRONIC SURFACE-STATES OF COO(100) - AN ELECTRON-ENERGY-LOSS STUDY

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