Low-energy vibrational dynamics of ultrathin organic adlayers: alkanethiols adsorbed on copper

The low-energy vibrational dynamics of alkanethiol monolayers adsorbed on two copper single-crystal surfaces [Cu(110), Cu(100)] has been measured by means of high-resolution inelastic helium-atom scattering. For the physisorbed phase of the two thiols studied [C2H5SH and C7H15SH], a characteristic mode of subthermal energy at about 6 meV was obtained which is attributed to the external vibration of the entire molecule perpendicular to the surface. In contrast, no distinct low-energy vibrations could be observed for the chemisorbed thiolate species, which forms well-ordered monolayer structures.