Monte Carlo simulation of reactive and non-reactive desorption of molecules from solid surfaces

Abstract

A Monte Carlo simulation scheme is developed to study thermal desorption of gas molecules from mixed adlayers containing two species where reactive and non-reactive desorption coexist. The effects of non-reactive desorption on the spectra of reactive desorption are investigated. The lateral interactions among adatoms, adsorbate islanding, surface diffusion, initial surface coverage and site energies are taken into account. It is observed that in a wide range, when the reaction energy of two species occupying adjacent sites is lower than the site energy of the old species, the non-reactive desorption has little or very limited effect on the spectrum of the reactive desorption, and only when the activation energy is quite close to the site energy of the old species does the non-reactive desorption affect the spectrum of reactive desorption, along with the initial coverage and the lateral interactions among adatoms. This indicates that earlier studies that separate reactive desorption from non-reactive desorption can be explained by the difference between the reaction energy and the site energies instead of the ‘time scale difference’ assumption. However, it is found that the non-reactive desorption terminates the reactive desorption earlier (cutting off the tail) in general cases and, when the non-reactive desorption is strongly effective, an increase of the coverage of B widens the spectrum of the reactive desorption instead of narrowing the spectrum, as in general cases, due to the effect of preventing early exhaust of the A species.