Accommodation of Gases at Rough Surfaces

A simple numerical model is proposed to compute the energy and momentum accommodation of molecules scattered from highly corrugated, disordered surfaces. The model is an extension of the "washboard model", which assumes that the component of the molecule's momentum parallel to the local surface tangent is conserved on impact and the normal component is altered by a hard, elastic collision with a moving surface "cube" with an adjustable effective mass. The surface is represented by Gaussian hills and valleys of random location and height. In contrast to the washboard model, the current model is fully three-dimensional and includes in-plane and out-of-plane scattering as well as trapping-desorption. In addition, it can be applied to highly corrugated surfaces and does not invoke a regular, periodic topography. This increased realism comes at the expense of an analytical solution; numerical simulations must be performed. We develop a very efficient procedure for carrying out the simulations. We test the model by comparing detailed angular and velocity scattering distributions for Xe scattering from a Pt(111) surface with those obtained by realistic molecular dynamics simulations of the same system. We then apply the model to the accommodation of H-2, N-2, CO, and CO2 on surface materials employed on vehicles in low Earth orbit. The model is capable of accurately reproducing the results of experimental measurements on these highly corrugated surfaces.