T. Hertel, E. Knoesel, E. Hasselbrink, M. Wolf, and G. Ertl
The unoccupied electronic states of CO/Cu(111) have been studied with two-photon photoemission (2PPE). For excitation with 3.5 eV < hv < 4.5 eV the kinetic energy of photoelectrons emitted from a CO induced state is found to shift as delta(Ekin) = 1delta(hv). This gives evidence for emission from a normally unoccupied state, located (3.35 ± 0.1) eV above the Fermi level, which is transiently populated in the 2PPE process. Based on the polarization dependence this state is identified as the 2¼* affinity level of CO adsorbed on Cu(111). For the 2¼* derived state very little dispersion is found along the gamma K direction. The results are discussed in the context of the 2¼* resonance model for CO chemisorption on metals.