Abstract

Knowledge of the rates and mechanisms of vibrational relaxation of molecules adsorbed on surfaces is essential to understand the dynamics of surface processes such as sticking, desorption, and surface chemical reactions. For metallic surfaces, an important outstanding question is the role played by the bulk metal conduction electrons in the damping of adsorbate vibrations. Theoretical studies have predicted lifetimes for the stretch vibration of chemisorbed CO of less than 10 ps as a result of efficient electron hole pair damping on metal surfaces [1,2]. This lifetime is one to two orders of magnitude shorter than that expected for relaxation directly to surface phonons [3]. Experimentally, a lower limit on the vibrational T₁ lifetime of ≈2 ps for CO on single crystal surfaces has been inferred from infrared absorption bandshape measurements [4,5]. However, the inability of frequency domain experiments to distinguish among the contributions to the measured bandwidth from depopulation, dephasing and inhomogeneity makes the interpretation ambiguous [6,7]. In order to clearly resolve this issue, direct measurements of the vibrational lifetimes of CO chemisorbed on metal surfaces are required.

© 1989 Optical Society of America