Bonding state formed between adsorbed benzene and Cu(111)

Two-photon photoemission spectroscopy of a benzene-adsorbed Cu(111) surface has revealed that adsorption-induced occupied and unoccupied electronic states are formed at −0.23 and +1.0 eV with respect to the Fermi level. The states are assigned to be due to bonding and antibonding orbitals formed by hybridization of benzene π-orbital and copper s,p orbitals. The bonding state, which is difficult to identify with UPS, is greatly enhanced by resonance with the image potential state. The photoemission intensity of the bonding state has been measured as a function of photon energy. The excitation efficiency curve shows that the resonance enhancement factor is more than 100, and the resonance width is less than 0.15 eV. However, bulk states or the occupied copper surface state showed no resonance with the image state of a benzene-covered surface area.