Molecular vibrational energy relaxation at a metal surface: Methyl thiolate on Ag(111)

The lifetime of the first excited level of the symmetric C-H stretching mode of CH₃S chemisorbed on a Ag(111) surface was measured by picosecond vibrational spectroscopy. A biexponential decay ({~}3- and 63-ps lifetimes at 300 K) was observed, with a substantial temperature dependence of the slow component. Both decay processes are assigned to intramolecular vibrational relaxation. The decay rates are 2 orders of magnitude too fast to be explained by electron-hole-pair damping by the metal substrate.