Low-frequency adsorbate vibrational relaxation and sliding friction

We present a study of the damping (energy relaxation) of low-frequency adso rbate vibrational modes via one-phonon emission. The dependence of the damp ing rate on the size of the adsorbates, the adsorbate coverage, and the nat ure of the (nonphononic) adsorbate-adsorbate interactions has been studied. We show that different experimental probing techniques, e.g., inelastic he lium scattering or lime-resolved adsorbate response to substrate heating, p robe different damping rates. We also show that the energy relaxation rate may be strongly affected by (pure) dephasing processes. Experimental data a re presented and compared to the theoretical results. Finally, we show that the phononic friction vanishes for incommensurate layers sliding on solid surfaces and present a discussion about the nature of the surfaces used in recent measurement of the sliding friction on superconducting lead surfaces . [S0163-1829(99)07417-2].