CONTINUUM ELASTIC THEORY OF ADSORBATE VIBRATIONAL-RELAXATION

An analytical theory is presented for the damping of low-frequency ads orbate vibrations via resonant coupling to the substrate phonons. The system is treated classically, with the substrate modeled as a semi-in finite elastic continuum and the adsorbate overlayer modeled as an arr ay of point masses connected to the surface by harmonic springs. The t heory provides a simple expression for the relaxation rate in terms of fundamental parameters of the system: gamma=m omega(0)(2-)/A(c) rho c (T), where m is the adsorbate mass, <(omega)over bar>(0) is the measur ed frequency, A(c) is the overlayer unit-cell area, and rho and c(T) a re the substrate mass density and transverse speed of sound, respectiv ely. This expression is strongly coverage dependent, and predicts rela xation rates in excellent quantitative agreement with available experi ments. For a half-monolayer of carbon monoxide on the copper (100) sur face, the predicted damping rate of in-plane frustrated translations i s 0.50X10(12) s(-1), as compared to the experimental value of (0.43+/- 0.07)X10(12) s(-1). Furthermore it is shown that, for all coverages pr esently accessible to experiment, adsorbate motions exhibit collective effects which cannot be treated as stemming from isolated oscillators . (C) 1998 American Institute of Physics. [S0021-9606(98)51803-X].