Sum-frequency spectroscopy in total internal reflection geometry: signal enhancement and access to molecular properties

The angular dependence of the sum-frequency signal radiated in a total internal reflection geometry is studied both theoretically and experimentally for a monolayer of octadecyl trichlorosilane chemisorbed upon CaF2. A predicted signal enhancement of 2 orders of magnitude is verified experimentally. The line amplitudes of the CH-stretching modes of the terminal methyl group depend strongly on the polarization geometry, the molecular hyperpolarizability, and the angles of incidence of the fundamental waves. By exploiting both the angular and the polarization dependence of the line amplitudes, one can obtain the molecular properties unambiguously. The hyperpolarizability tensor elements of the terminal methyl group are determined directly from the sum-frequency spectra.