Unoccupied adsorbate states of COCu(111) analyzed with two-photon photoemission

The unoccupied electronic states of the CO-(1.4 × 1.4) overlayer on Cu(111) have been studied with two-photon photoemission (2PPE). For excitation with 3.5⩽hv⩽4.5 eV the kinetic energy of photoelectrons emitted from a CO induced state is found to shift as ΔEkin= 1 · Δhv. This provides 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 of the adsorbate induced feature this state is identified as being derived from the 2π* affinity level of CO adsorbed on Cu(111). No dispersion of the CO2π* level is observed along TK