On the attenuation of anharmonic adsorbate vibrations

Adsorbate vibrations on crystal surfaces are discussed as candidates for the existence of breathers in microscopic solid state physics. The attenuation of coherently excited adsorbate vibrations due to their anharmonic interaction with the quantum and thermal fluctuations of other phonon modes in the system is investigated. On the basis of ab initio calculations of harmonic and anharmonic force constants for the system Si(1 1 1) :H, the lifetime of the zone-center Si-H stretching mode is determined as function of temperature. The results compare well with available experimental data and yield a first estimate for the lifetime of localized modes with amplitudes smaller than the mean square displacements due to thermal and zero-point motion. A theoretical treatment of the attenuation of nonlinear localized adsorbate modes is given in the opposite regime, i.e. large driven amplitudes as compared to the thermal and zero-point motion. (C) 1998 Elsevier Science B.V.