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Picosecond infraredÈvisible sum-frequency generation (SFG) surface vibrational spectroscopy was applied for in situ monitoring of chemisorbed CO on a polycrystalline platinum foil at room temperature. The dependence of the SFG spectra on the CO gas-phase pressure was investigated in the range mbar. From p CO \ 10~8È1000 the measured SFG spectra, frequencies of the CO vibrationally resonant contribution were determined as a function of CO pressure. At low CO pressures (10~8È10 mbar) a single vibrational band with center frequency in the range 2091È2099 cm~1 was obtained which is characteristic of stretching vibrations of CO terminally adsorbed on Pt atoms of low-index (100)-, (110)-and ( 111)-type planes. At higher pressure mbar) the (p CO P 50 appearance of new "" low-frequency ÏÏ CO surface species was observed which dominate the SFG spectra at CO pressures above 300 mbar. The low-frequency spectral feature was found to be completely reversible and reproducible with variation of the CO pressure. The pressure-dependent change in the SFG spectra suggests formation of platinum carbonyl binary complexes at surface structures resulting from a reversible PtÈ(CO) 2 CO adsorbate-induced displacive reconstruction of the Pt substrate at high CO pressures.

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NO Adsorption / Catalysis / Sum-Frequency Generation / Low-Energy Electron DiffractionIn-situ sum-frequency generation (SFG) surface vibrational spectroscopy for different polarisation arrangements has been employed along with low-energy electron diffraction (LEED) measurements and kinetic modelling calculations to study NO adsorption on Pt(111) as a function of NO gas-phase pressure (p NO ϭ 10 Ϫ9 Ϫ10 Ϫ4 mbar) and substrate temperature (T s ϭ 300Ϫ400 K). The observation of a NO stretching vibrational band with a frequency of 1724 cm Ϫ1 with different signal intensities for ssp and ppp polarisation combinations indicated the presence of tilted NO species at high NO coverages (θ Ն 0.5). At lower coverages (0.5 Ͼ θ Ͼ 0.25) the adsorption geometry was found to change towards upright NO giving rise to a vibrational band with slightly lower frequencies of 1716Ϫ1720 cm Ϫ1 . LEED studies demonstrated that under adsorption/desorption equilibrium conditions up to saturation coverage NO adsorption leads to the formation of ordered adsorbate structures with 2ϫ 2 periodicity. The detailed analysis of the pressure and temperature dependence of the LEED and SFG surface vibrational data is consistent with a NO adsorption mechanism involving the successive formation of Pt(111)ϩp(2ϫ2)-NO (θ ϭ 0.25), Pt(111)ϩ(2ϫ2)-2 NO (θ ϭ 0.5) and Pt(111)ϩ(2ϫ2)-3 NO (θ ϭ 0.75) adsorbate structures as the NO pressure is increased from 10 Ϫ9 to 10 Ϫ4 mbar. A tentative binding site assignment for the two higher-coverage structures with θ ϭ 0.5 and 0.75 is proposed and discussed in the light of the present and previous experimental data.

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