Low-frequency adsorbate vibrational relaxation and sliding friction

We present a study of the damping (energy relaxation) of low-frequency adsorbate vibrational modes via one-phonon emission. The dependence of the damping rate on the size of the adsorbates, the adsorbate coverage, and the nature of the (nonphononic) adsorbate-adsorbate interactions has been studied. We show that different experimental probing techniques, e.g., inelastic helium scattering or time-resolved adsorbate response to substrate heating, probe different damping rates. We also show that the energy relaxation rate may be strongly affected by (pure) dephasing processes. Experimental data are 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.