VIBRATIONAL SUM-FREQUENCY SPECTROSCOPY OF PHASE-TRANSITIONS IN LANGMUIR FILMS AND 3-COLOR PUMP-PROBE STUDIES OF CHEMISORBED MOLECULES

chi((2))-processes have proven to be very useful for studying surfaces and interfaces. Due to the symmetry breaking at the boundary of centr osymmetric media where chi((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 chi((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 t he SF spectra the orientation and conformation of the adsorbed molecul es is derived. For stearic acid a direct transition from the liquid-co ndensed to the gaseous phase is found, which agrees with independent s urface tension measurements. Octadecylamine shows a liquid-expanded ph ase characterized by molecules in gauche conformation. Using an extens ion of the technique with two independently tunable picosecond IR puls es combined with visible upconversion. we also study vibrational energ y relaxation and intramolecular energy transfer at surfaces. Prelimina ry data are presented on the vibrational energy decay and energy trans fer of a terminal methyl group.