Chemisorbed films of octadecyltrichlorosilane (OTS) and ethyltrichlorosilane (ETS) at the interface calcium fluoriderair have been studied by vibrationally resonant IR pump IR-vis probe sum-frequency (SF) spectroscopy. Using a total internal reflection geometry, a signal enhancement of more than two orders of magnitude compared to the conventional setup is achieved. This enhancement in combination with two independently continuously tunable in- dielectric surfaces. A fast energy exchange between the CH3-stretching modes detected by SF spectroscopy occurs within a few picoseconds. A two-exponential decay of the CH3-stretching modes is observed: the energy relaxation is fast with time constants in the picosecond range, while the ground sate is repopulated in approximately 100ps. of a SF-active mode can be achieved, even if the pumped mode does not directly contribute to the SF spectrum. This weakening of the selection rules in IR pump IR-vis probe SF spectroscopy is demonstrated for the methylene stretching vibrations of OTS.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.