Vibrational Sum Frequency Spectroscopy Studies at Solid/Liquid Interfaces: Influence of the Experimental Geometry in the Spectral Shape and Enhancement

Abstract

The influence of the experimental geometry, specifically the angles of incidence (AOI) of the exciting beams, on the enhancement of the vibrational sum frequency spectroscopy (VSFS) spectra has been systematically investigated, particularly when approaching total internal reflection (TIR) conditions. Theoretical simulations of the spectral intensity as a function of the AOI and infrared wavelength at three different polarization combinations were critically compared to experimental data obtained from a hydrophobized silica/water interface. The spectra were recorded using a newly established femtosecond VSFS spectrometer designed to allow rapid and accurate changes of the angles of incidence. At the most favorable AOI, when the IR, visible, and/or emitted sum frequency beams were close to TIR conditions, the spectral enhancement could be up to several orders of magnitude. The effect from the IR beam was however less pronounced due to absorption from the aqueous phase. Furthermore, the relative enhancement of the spectral features across the CH and OH stretching regions varied significantly depending on the angles of incidence chosen. The uneven enhancement is a consequence of the large dispersion in the IR refractive index of water. Although the theoretical simulations qualitatively predict this trend, the quantitative agreement with the experimental data is poor. The simulations underestimate the amplification in the free OH range by 2–4 times as the IR beam approaches TIR, and overestimates it as the visible beam approaches the critical angle. These observations may suggest a need to review the fundamental equations describing the angular dependence of the SF intensity. The practical advantages of selecting specific angles of incidence, as well as the expected variations for two other frequently encountered solid/liquid interfaces (CaF2/water and sapphire/water), are also discussed.