The coverage dependence of the pre-exponential factor for desorption

The pre-exponential factor for desorption υ₀ is seldom measured as other than low surface coverage θ and is typically assumed to be independent of θ. Variations in υ₀ with θ are critically important in describing surface equilibrium and rate processes and in testing theories of desorption. It has often been overlooked that the functional depedence of υ₀ on θ may be readily determined from thermodynamic data used to determine isosteric heats of adsorption if the sticking coefficient S(θ) is known. We present υ₀(θ) values for approximately 45 adsorption species calculated from or reported in the literature for approximately adsorption systems. It is found that variations in υ₀ by factors of 10⁶ are often observed. In approximately half of these examples υ₀ decreases by a factor of 10³ or more and in the rest of the systems υ₀ is constant or decreases by less than 10³; in very few cases does υ₀ increase significantly with coverage. Current theoretical models for υ₀ are briefly reviewed, and it is shown that none predicts such large coverage variations. However, analysis of low coverage models suggest that adsorbate-induced changes in the surface phonon spectrum may play a critical role.