We have examined the effects of collisions among the molecules desorbing from solid surfaces by means of Monte Carlo simulations, and have identified the conditions under which and to what extent these collisions would influence the experimentally observed product distributions. By simulating the experiment performed by Cowin et al. [Surf. Sci. 7 8, 545 (1978)] on the laser induced desorption of D₂ from tungsten, we have found that at high coverages each desorbate makes on average 2.9 collisions which decreases to no collisions at very low coverages. These collisions affect the product distribution at high coverages to such an extent that even if the nascent desorbed flux is under thermal equilibrium, these post‐desorption collisions could lead to nonequilibrium distributions. The effect of the post‐desorption collisions is influenced by the rate of heating the surface and the kinetics of the desorption process.

The effects of gas-phase collisions in mixtures of gases rapidly desorbed from surfaces are studied using direct Monte Carlo techniques. The results are compared with the effects observed in the desorption of pure gases under similar conditions. The translational energy distribution of the desorbed particles are found to deviate from the Boltzmann distribution and are found to be well represented by ellipsoidal Boltzmann distributions. In this respect the rapid desorption process is found to have similarities to the expansion of gases in nozzle sources. The influence of mass, internal degrees of freedom, and surface coverage of the adsorbates on the focusing, accelerating, and cooling effects due to gas-phase collisions are analyzed. The presence of molecules with active internal degrees of freedom is found to increase the average number of collisions experienced by the rapidly desorbed molecules. However, the influence of this increased number of collisions on the focusing effects due to gas-phase collisions is less pronounced compared to the focusing effects due to collisions between the desorbed atoms. In a gas mixture containing molecules as the minor constituents (10%) and atoms as the major constituents (90%), atoms are found to be more focused towards the surface normal than the molecules and the mean translational energies of the molecules are found to be less than those calculated in the desorption of pure molecules under similar conditions. The presence of atoms in the desorbed gas mixture is found to increase the most probable speed of the desorbing molecules and this accelerating effect increases with decrease in the mass of the coadsorbed atoms. The light atoms are found to be more efficient than heavy atoms in cooling the internal degrees of freedom.

The influence of postdesorption collisions on the experimentally measured angular and energy distributions of the rapidly desorbed molecules is investigated using direct Monte Carlo simulation procedures. By simulating the desorption of NO from LiF(100), we find that the postdesorption collisions have a significant effect on the final angular, rotational energy, and translational energy distributions of the desorbed molecules.

A variational transition‐state theory study of the recombination/desorption rate of H₂ from a Si(111) surface using a previously described Monte Carlo procedure [J. Chem. Phys. 8 3, 1389 (1985)] is reported. The potential‐energy surface is expressed as the sum of a lattice potential, a lattice‐adatom interaction term, and an adatom–adatom interaction. Keating’s formulation with the parameters suggested by Weber is used for the lattice potential. The adatom–lattice term is written as a pairwise sum of 60 Morse potentials each multiplied by a hyperbolic switching function that limits each absorption site to one bond. The adatom–adatom interaction is the product of an H₂ Morse potential and a switching function that attenuates the H–H interaction as the Si–H bonds form. The parameters of the potential are adjusted to fit the results of hydrogen atom–silicon cluster calculations, the experimental and theoretical results for the H₂ insertion barrier into Si and SiH₂, and the measured H₂(g) bond dissociation energy, fundamental frequency, and equilibrium bond distance. The minimum‐energy path is obtained using a Monte Carlo random walk procedure with importance sampling. The potential surface predicts a 2.52 eV barrier for H₂ recombination/desorption and a 0.182 eV barrier to the back reaction. Variational rate calculations are carried out by expressing the dividing surface as a linear combination of the recombination/desorption coordinates and minimizing the computed flux across the surface with respect to the expansion coefficients using a partial grid search. An Arrhenius plot of the minimized flux yields an activation energy and a frequency factor of 2.41 eV and 0.202 cm² s, respectively. This activation energy is in good agreement with one reported experimental value and is 0.60 eV greater than that found in two other experiments. The frequency factor lies in the middle of the range of the reported experimental values.