Comparison of diffusion theory adsorption and desorption rate constants with experimental lifetimes

Abstract

Stimulated by the success of the diffusion theory for nonequilibrium gas phase recombination and dissociation, an analogous theory for nonequilibrium adsorption and desorption has been formulated. Experimental adsorption and desorption lifetimes are compared with the resulting steady state adsorption, and desorption rate constants. The goal of this comparison is to obtain gas-surface potential parameters for each adatom-substrate species combination for which data is available. The adatom energy distribution is obtained from a diffusion equation approximation to the governing master equation. The energy transition kernel is developed using an harmonic potential in the soft-cube model for the gas-surface collision. It is found that the pre-exponential factor in the desorption lifetime may be represented by a power series in the nondimensional potential well depth, δ = D / kTg. Gas surface potential parameters are derived for cadmium, gallium, titanium, cesium, silver, and rubidium on tungsten by comparison with desorption lifetimes. Adsorption lifetimes are predicted for these species. Good agreement is obtained with experimental measurement of the adsorption lifetime of xenon on tungsten. Adsorption and desorption lifetimes for xenon, krypton, and argon on tungsten; xenon, argon, and neon on gold; xenon, krypton, and argon on platinum; and xenon, krypton, and argon on silver are predicted over a wide range of gas and surface temperatures for comparison with future experiments.