Pressure dependence (10-8-1000 mbar) of the vibrational spectra of CO chemisorbed on polycrystalline platinum studied by infrared-visible sum-frequency generation

Picosecond infrared-visible 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 p(CO)=10(-8)-1000 mbar. From the measured SFG spectra, frequencies of the CO vibrationally resonant contribution were determined as a function of CO pressure. At low CO pressures (10(-8)-10 mbar) a single vibrational band with center frequency in the range 2091-2099 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 (p(CO)greater than or equal to 50 mbar) the appearance of new "low-frequency'' 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 Pt-(CO)(2) platinum carbonyl binary complexes at surface structures resulting from a reversible CO adsorbate-induced displacive reconstruction of the Pt substrate at high CO pressures.