Total Internal Reflection Sum-Frequency Spectroscopy:  A Strategy for Studying Molecular Adsorption on Metal Surfaces

Total internal reflection sum-frequency spectroscopy (TIR−SFS) is shown to be capable of detecting molecules adsorbed on ultrathin gold films (≤10 nm) deposited on a sapphire prism. Octadecanethiol (ODT) and thiocyanate (SCN^-) were used as probe molecules in order to assess the usefulness of the approach. For ODT adsorbed on 5-nm Au films, SF signal enhancements of over an order of magnitude were observed with TIR−SFS compared to the standard external reflection geometry. While TIR−SF spectra were obtained for ODT on 5- and 10-nm Au films, no molecular signals were detected for 20-nm Au films. The C&tbd1;N stretch of SCN^- adsorbed on a 5-nm Au film was detected by TIR−SFS in the presence of either water or air. A theoretical model is presented to rationalize the different SF signal levels observed under various conditions. Future prospects of TIR−SFS for studying other oxide-supported metals are discussed, along with possible applications in the fields of heterogeneous catalysis and electrochemistry.