VIBRATIONAL-ENERGY RELAXATION OF A POLYATOMIC ADSORBATE ON A METAL-SURFACE - METHYL THIOLATE (CH3S) ON AG(111)

The lifetime of the first excited level of the symmetric C-H stretching mode of methyl thiolate (CH3S) bonded to Ag(1 1 1) is measured by populating the level with a picosecond infrared pulse and probing the population by transient sum frequency generation spectroscopy. The population transient shows a biexponential decay across the experimental temperature range from 110 to 380 K. The fast decay component has a lifetime of 2.5-3 ps at all temperatures. The slow relaxation component lifetime varies from 55 ps at 380 K to 90 ps at 110 K. Neither relaxation component shows decay rates that are compatible with direct energy transfer to phonons or electron-hole pairs of the metal substrate. Both relaxation components are instead assigned to intramolecular energy transfer to excited vibrational levels of other vibrational modes of the molecule.