Observation of the broadening and shift of the frustrated translation vibrational mode of CO on Cu(001) by high resolution helium atom scattering

The lifetime broadening of the frustrated translation (T) mode parallel to the surface of isolated CO molecules [less than or similar to 6% of a c(2x2) coverage] chemisorbed on Cu(001) has been studied with high resolution inelastic helium atom scattering as a function of the surface temperature and the parallel momentum transfer. The T excitation peak shows a temperature dependent peak shift and a broadening. The latter is attributed to a quartic anharmonic potential contribution described by a spectroscopic constant chi(e) = -0.0085 +/- 0.O008. The extrapolated T-s = 0 K peak broadening is then gamma(0) = 85 +/- 5 mu eV, corresponding to a vibrational lifetime tau = 8 +/- 1 ps. We attribute this lifetime to substrate phonon mediated damping and electronic damping. (C) 1996 American Institute of Physics.