Vibrational Energy Relaxation in a Molecular Monolayer at a Metal Surface.

Direct measurements of molecular vibrational energy transfer solid surfaces have not been available, despite the fundamental importance of vibrational energy relaxation in all surface chemical dynamics, and despite considerable theoretical interest. We report picosecond time-resolved measurements of vibrational energy relaxation from upsilon=1 to upsilon =0 for C-H stretching modes of the terminal methyl group in a Cd stearate Langmuir- Blodgett monolayer on an evaporated silver film. This is the first direct measurement of vibrational energy relaxation for molecular adsorbates at a bulk metal surface. The experiments utilized infrared-visible sum frequency vibrational spectroscopy as a picosecond monolayer-sensitive probe of vibrational dynamics. Multicomponent decay processes with lifetimes of 3 ps to >1 ns indicate complex intramolecular vibrational energy transfer processes in these ordered monolayer films, which may be different than for similar molecules in liquids.