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Abstract:
Photoinduced excitations of normally unoccupied electronic states of adsorbates are investigated with two-photon photoemission (2PPE). We report 2PPE spectra from an ordered (1.4 × 1.4)-CO layer on Cu(1 1 1) for excitation with 3.5 eV ≤ hv ≤ 4.5 eV photon energy. The energetic position, Ekin, of a CO induced state is found to shift as ΔEkin = 1 × Δhv. This provides evidence 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 2π* affinity level of chemisorbed CO on Cu(1 1 1) which is temporarily populated in the 2PPE process.
