DESORPTION OF VIBRATIONALLY EXCITED ADSORBATES IN COMPETITION WITH RELAXATION - A CLASSICAL PICTURE

The competition between desorption and relaxation of vibrationally excited CO adsorbed on a series of model surfaces is examined theoretically using a classical mechanical description of the dynamics. The analogy between these processes and the vibrational predissociation of van der Waals molecules in the gas phase is emphasized. This study, which is restricted to a collinear arrangement of the adsorbate and surface atoms explores the effect of a systematic variation of the parameters of the simple model systems. The parameters varied include the surface Debye frequency and the strength of the adsorbate-surface interaction. Rate constants for the competing processes of predesorption, relaxation, and thermal desorption of both excited and relaxed adsorbate molecules are extracted as a function of the parameter values. Surface vibrations in models of NaCl and Si were found to accelerate the classical predesorption rate compared to the case of a stationary surface. This effect can be rationalized in terms of an impulsive collision mechanism. Raising the surface Debye frequency was found to enhance relaxation at the expense of predesorption. Mode mixing between adsorbate and surface modes appears to be a more important factor in this regard than Fermi resonance, or mode matching. Thermal desorption was not found to be a significant process in these classical calculations. Comparisons with parallel quantal calculations are made in the following paper. © 1990 American Institute of Physics.