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 (less th an or equal to 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 en hancements of over an order of magnitude were observed with TIR-SFS compare d to the standard external reflection geometry. While TIR-SF spectra were o btained for ODT on 5- and 10-nm Au films, no molecular signals were detecte d for 20-nm Au films. The C=N stretch of SCN- adsorbed on a 5-nm Au film wa s 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 ox ide-supported metals are discussed, along with possible applications in the fields of heterogeneous catalysis and electrochemistry.