Electronic excitations on clean and adsorbate-covered oxide surfaces

We have studied electronic excitations at the surfaces of NiO (100), Cr₂O₃(111), and Al₂O₃(111) thin films with Electron Energy Loss Spectroscopy (EELS). On NiO (100) we observe surface electronic excitations in the energy range of the band gap which shift upon adsorption of NO. Ab initio cluster calculations show that these excitations occur within the Ni ions at the oxide surface. The (111) surface of Cr₂O₃ is characterized by distinct excitations which are also strongly influenced by the interaction with adsorbates. Temperature-dependent measurements show that two different states of the surface exist which are separated by an activation energy of about 10 meV. For Al₂O₃(111) we present data for a CO adsorbate. The oxide is quite inert with respect to CO adsorption as indicated by desorption temperatures between 38 K and 67 K. Due to the weak interaction with the substrate the a³II valence excitation of CO shows a clearly detectable vibrational splitting which has not been observed previously for a CO adsorbate in the (sub)monolayer coverage range. For several different adsorption state the lifetimes of the a³II state could be estimated from the halfwidths of the loss peaks, yielding values between 10^–15 s for the most strongly bound species and 10^–14 s for the CO multilayer.