Molecular vibrational energy relaxation at metal surfaces by picosecond infrared-visible sum spectroscopy

Time-domain experiments have been conducted to establish vibrational energy relaxation mechanisms for adsorbates at surfaces, using transient sum frequency generation (SFG). An intense resonant (IR) pump pulse vibrationally excites molecules at the surface. The subsequent population relaxation is followed by monitoring the SPG signal from a delayed pair of synchronized picosecond IR and visible probe pulses. The transient SFG experiment probes vibrational level population changes in a manner similar to familiar transient absorption experiments; the vibrationally resonant SFG nonlinear susceptibility is proportional to the population difference between the lower and upper levels of the transition. The descriptions of absorption and SFG as probes of population changes are quite similar in perturbation theory. The advantage of transient SGF measurements for surface studies is their sensitivity. Studies of methyl (CH₃) groups in both large and small molecules at surfaces and methyl thiolate (CH₃S) on a Ag(111) surface are reported.