Vibrational sum-frequency spectroscopy of phase transitions in Langmuir films and three-colour pump-probe studies of chemisorbed molecules

χ⁽²⁾-processes have proven to be very useful for studying surfaces and interfaces. Due to the symmetry breaking at the boundary of centrosymmetric media where χ⁽²⁾ vanishes in the bulk, only a very thin layer at the interface contributes to the three-wave mixing process. Exploiting resonant enhancement of the nonlinear coupling, vibrational sum-frequency (SF) spectroscopy combines the advantages of a surface-specific χ⁽²⁾-process with mode-specific methods like IR and Raman spectroscopy. We present novel data on Langmuir films of stearic acid and octadecylamine on water at various surface concentrations. From the SF spectra the orientation and conformation of the adsorbed molecules is derived. For stearic acid a direct transition from the liquid-condensed to the gaseous phase is found, which agrees with independent surface tension measurements. Octadecylamine shows a liquid-expandea^⊙ phase characterized by molecules in gauche conformation. Using an extension of the technique with two independently tunable picosecond IR pulses combined with visible upconversion, we also study vibrational energy relaxation and intramolecular energy transfer at surfaces. Preliminary data are presented on the vibrational energy decay and energy transfer of a terminal methyl group.