Vibrational energy relaxation of methyl thiolate/Ag(111) by picosecond IR-visible sum spectrosopy

Vibrational energy relaxation of the symmetric C-H stretching mode of methyl thiolate on a Ag(l 1 1) surface is measured by picosecond infrared-visible sum frequency generation. Vibrational relaxation lifetime components of ~3 PS and 63 PS are observed at 300 K. The long lifetime component shows a moderate temperature dependence. Both population relaxation components are assigned to intramolecular energy transfer on the basis of comparisons with other measurements and the predicted temperature dependence of intramolecular relaxation rates. Direct energy transfer to electronic or vibrational degrees of freedom in the substrate is not found to be important for this vibrational mode.