Damping of Molecular Motion on a Solid Substrate: Evidence for Electron-Hole Pair Creation

The damping of molecular motion relative to a solid substrate was investigated by high-resolution vibrational spectroscopy in the far-infrared regime. For octane adsorbed on a hydrogen-passivated Ru(0001) surface the experimental linewidths observed by He-atom scattering for the frustrated translation normal to the surface are in accord with a damping by phonon emission, but a strong broadening is found for the clean Ru substrate. This enhanced friction is related to an increase in the rate of electron-hole $\left(e-h\right)$ pair creation due to the presence of hybrid metal-molecule electronic states which are absent for the hydrogen passivated surface. Data from near-edge x-ray absorption spectroscopy provide direct evidence for these differences in electronic structure. Results from IR spectroscopy strongly suggest that the soft C-H bands observed for these systems are also related to the formation of these hybrid electronic states.