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

Abstract

Sum‐frequency spectroscopy (SFS) was used in an attempt to detect the platinum–carbon vibration of CO adsorbed on Pt(111). The international free‐electron laser FELIX at the FOM Institute, Rijnhuizen, provided the required tunable far‐infrared (19–23 µm) source, while complementary measurements in the C–O stretch region (4.7–5.1 µm) were performed at the University of Oxford with a conventional nanosecond laser system. Ordered Pt(111) surfaces were prepared by the H2/O2 flame annealing approach and CO monolayers were produced by exposure of the Pt crystal to gaseous CO in a flow reactor. The monolayers were characterized by sum‐frequency (SF) measurements of the vC-O vibrational frequency. The CO adsorbed primarily in the terminal (atop) configuration, with a vC-O frequency of around 2078 cm−1. In the far‐IR region, the non‐resonant background from the Pt substrate could readily be detected by SFS, but there was no evidence for the vPt-CO mode. Direct laser‐induced desorption and thermal desorption of CO are unlikely under the experimental conditions. It is therefore probable that the intrinsic cross‐section of the Pt–CO mode is too low for easy detection by SFS. The implications for the use of SFS to detect metal–adsorbate vibrational modes are discussed in light of these findings.