Two-photon photoemission study ofCO/Pt(111)

Abstract

Unoccupied electronic states induced by CO chemisorbed on a Pt(111) surface have been investigated by two-photon photoelectron (2PPE) spectroscopy. The 2PPE spectra obtained by excitation with4.3eV⩽hv ⩽ 5.4eV indicate that photoemission peaks prominent in the 2PPE spectra from the CO-covered Pt(111) surface originate from normally unoccupied CO-induced states above the Fermi level ( E f ), which are transiently occupied in the 2PPE process. The energy of the CO-induced unoccupied state depends on the adsorption site of CO. Below 0.17 ML where CO is adsorbed at the on-top site, the CO-induced state is located at 4.5 eV above E f . With an increase in the coverage to 0.5 ML, where CO is adsorbed at the bridge site in addition to the on-top site, a new photoemission peak at 4.2 eV grows. This peak is assigned to photoemission from the unoccupied state of bridge-bonded CO. From the polarization dependence of the photoemission peaks, these unoccupied states are assigned to the CO2π * derived states hybridized with the Pt 5d-band.