Abstract

Energy transfer at surfaces is important in physical processes such as sticking, desorption, diffusion, and the chemistry of oxidation, catalysis, and electronic materials processing. Early information on rates and mechanisms of vibrational energy dissipation came from theory or were inferred from linewidths of optical spectra. We have used tunable subpicosecond IR laser pulses in pump-probe experiments to obtain time-resolved information about the vibrational energy relaxation time (T₁) and homogeneous dephasing time (T₂) for the high-frequency CO (v = 1) stretch mode of an ordered monolayer of CO on the surface of a Pt(111) single crystal. The observed Ti ≈ 4 ps at low temperatures is slower than would be inferred from the IR bandwidth for top-site CO. Transient IR spectra of the excited adlayer suggest that the CO (v = 1) → (v = 2) band is anharmonic- ally shifted from the fundamental v = 0 → v = 1 transition by only 4-5 cm⁻¹. These results are compared with the long values of T for OH, NH, or SiH stretch modes on nonmetallic surfaces, and to spectral data and time-resolved studies of very fast CO-CO coupling in metal carbonyl molecules and very short T₁ for CO (v = 1) chemisorbed on amorphous metal particles. These comparisons, as well as theory, suggest the important energy relaxation mechanism for CO (v = 1 )/Pt(111) is coupling of the vibrating dipole to metal free electrons, i.e., damping by electron-hole pair formation.

© 1990 Optical Society of America