UNOCCUPIED ADSORBATE STATES OF CO/CU(111) ANALYZED WITH 2-PHOTON PHOTOEMISSION

The unoccupied electronic states of the CO-(1.4 x 1.4) overlayer on Cu(111) have been studied with two-photon photoemission (2PPE). For excitation with 3.5 less-than-or-equal-to hnu less-than-or-equal-to 4.5 eV the kinetic energy of photoelectrons emitted from a CO induced state is found to shift as DELTAE(kin) = 1 . DELTAhnu. 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 2pi* affinity level of CO adsorbed on Cu(111). No dispersion of the CO 2pi* level is observed along GAMMABAR KBAR.