The microscopic origins of sliding friction: a spectroscopic approach

The nature of the mechanism governing the damping of molecular motion relative to a solid substrate has been investigated by a systematic study of molecular frustrated translations. Using scattering of thermal energy Heatoms, these vibrations located in the far-infrared regime (< 15 meV) have been determined for prototype lubricant molecules, octane and other saturated hydrocarbons, adsorbed on a variety of different substrates, namely hydrogen-terminated diamond C(111), Pb(111), clean and hydrogen passivated Ru(0001) as well as on Cu(111). In most cases the lifetime of the frustrated translational modes is in good agreement with that computed from theoretical models considering only phononic damping. Exceptions to this rule (Pb(111) and Ru(0001)) are discussed. The results suggest that generally electronic contributions to sliding friction are weak; only in cases with significant electronic coupling between molecule and substrate does electronic friction dominate phononic friction.