EXTERNAL VIBRATIONS OF HYDROCARBONS ON CU(100)

A search for external vibrations of hydrocarbons physisorbed on Cu(100 ) has been carried out with high-resolution inelastic He-atom scatteri ng. For the saturated hydrocarbons n-hexane, cyclohexane, n-octane, an d n-decane, the frustrated translation normal to the surface could be detected with energies of 6.8 meV, 7.3 meV, 7.0 meV, and 7.0 meV, resp ectively. A very similar value of 7.3 meV for the same vibration is ob served for the unsaturated hydrocarbon benzene. No vibrational modes c orresponding to frustrated translations parallel to the surface could be detected. (C) 1995 American Institute of Physics.