Analytical Model of Heterogeneous Atomic Recombination on Silicalike Surfaces

An analytic model to describe the heterogeneous recombination of a single atomic species on silicalike surfaces is developed. The theoretical investigation herein presented provides ready-to-use expressions for the surface atomic recombination probability γ obtained as a function of surface characteristics such as the densities of adsorption sites and the activation energies for the different elementary surface processes. The model takes into account physisorption, chemisorption, thermal desorption, surface diffusion, and both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) recombination mechanisms. The results are applied to the cases of nitrogen and oxygen recombination on silica and to oxygen recombination on Pyrex. However, since the derivation is kept in a very general form, it allows the exploration of several distinct limit cases and provides a deeper understanding of the underlaying surface kinetics. The dependence of the recombination probability with the wall temperature and with the gas pressure is studied in detail. It is found that γ can have a complex nonmonotonic behavior with the wall temperature, as a result of the competition between E-R and L-H recombination processes. The transition from first- to second-order recombinations (and vice-versa) with pressure is studied and debated.