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 a dsorbed on various metal surfaces by means of high-resolution He atom scattering. In the case of octane, the frustrated translation normal t o the surface (FTz) could be detected for all substrates with energies of between 6.6 and 7.2 meV. By evaluating the corresponding line widt h, the lifetime of this mode can be determined for the different subst rates. The results are compared to theoretical values, which where obt ained by computing the damping due to emission of phonons into the sub strate.