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 resolve d energy losses of 13.1, 12.9, and 12.7 meV were observed for the pure benzene (2 square-root 3 x 3) structure, the CO + C6H6 c(2 square-roo t 3 x 4)rect and (3 x 3) structures, respectively. As these are the fi rst experimental data for vibrations of a large adsorbed molecule in t his energy regime extended Hueckel calculations were used to identify the observed mode as the frustrated translation of the benzene molecul e parallel to the surface.