Prospects for detecting metal-adsorbate vibrations by sum-frequency spectroscopy

Sum-frequency spectroscopy (SFS) was used in an attempt to detect the plati num-carbon vibration of CO adsorbed on Pt(111). The international free-elec tron laser FELIX at the FOM Institute, Rijnhuizen, provided the required tu nable far-infrared (19-23 mu m) source, while complementary measurements in the C-O stretch region (4.7-5.1 mu m) were performed at the University of Oxford with a conventional nanosecond laser system. Ordered Pt(111) surface s were prepared by the H-2/O-2 flame annealing approach and CO monolayers w ere produced by exposure of the Pt crystal to gaseous CO in a flow reactor. The monolayers were characterized by sum-frequency (SF) measurements of th e nu(C-O) vibrational frequency. The CO adsorbed primarily in the terminal (atop) configuration, with a nu(C-O) frequency of around 2078 cm(-1). In th e far-IR region, the non-resonant background from the Pt substrate could re adily be detected by SFS, but there was no evidence for the nu(Pt-CO) mode. Direct laser-induced desorption and thermal desorption of CO are unlikely under the experimental conditions. It is therefore probable that the intrin sic cross-section of the Pt-CO mode is too low for easy detection by SFS. T he implications for the use of SFS to detect metal-adsorbate vibrational mo des are discussed in light of these findings.