Vibrational relaxation lifetime of a physisorbed molecule at a metal surface.

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1-3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v = 1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49 +/- 3 ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances