Damping of external molecular vibrations at surfaces: Alkanes on Cu(111) and Pb(111)

We have investigated the low-energy external vibrations of n-alkanes adsorbed on various metal surfaces by means of high-resolution He atom scattering. In the case of octane, the frustrated translation normal to the surface (FTz) could be detected for all substrates with energies of between 6.6 and 7.2 meV. By evaluating the corresponding line width, the lifetime of this mode can be determined for the different substrates. The results are compared to theoretical values, which where obtained by computing the damping due to emission of phonons into the substrate.