Observation of the low-energy external vibrations of benzene on Rh(111)

The low frequency vibrations of benzene molecules adsorbed on Rh(111) have been investigated with inelastic He atom scattering. Well resolved energy losses of 13.1, 12.9, and 12.7 meV were observed for the pure benzene (2 {}3 ×{}3) structure, the CO+C₆{H}₆$ c(2 {}3 ×{}4) rect and (3×{}3) structures, respectively. As these are the first experimental data for vibrations of a large adsorbed molecule in this energy regime extended Hueckel calculations were used to identify the observed mode as the frustrated translation of the benzene molecule parallel to the surface.