ELECTRONIC EXCITATIONS OF ADSORBATES ANALYZED WITH 2-PHOTON PHOTOEMISSION - CO CU(111)/

Photoinduced excitations of normally unoccupied electronic states of a dsorbates are investigated with two-photon photoemission (2PPE). We re port 2PPE spectra from an ordered (1.4 x 1.4)-CO layer on Cu(111) for excitation with 3.5 eV less than or equal to hv less than or equal to 4.5eV photon energy. The energetic position, E(kin), of a CO induced s tate is found to shift as Delta E(kin) = 1 x Delta hv. This provides e vidence for emission from a normally unoccupied state, located (3.3 +/ - 0.2) eV above the Fermi level. Based on the polarization dependence of the emission from this adsorbate-induced state it is identified as the 2 pi affinity level of chemisorbed CO on Cu(111) which is tempora rily populated in the 2PPE process.