Gas-Surface Desorption and Scattering Processes: Development and Application of the Random Corrugation Model

In the first part of this work, an ad hoc mathematical analysis of mode selective desorption of Hz from Si(111)1x1 is presented. Recent experiments have shown a decidedly non-thermal preferential desorption of D2 over D2 and HD. The system is examined via percolation theory and also with a coupled harmonic oscillator treatment. Despite much excitement upon the original publication of these experimental findings, there remain unanswered considerations about the underlying molecular mechanics.;In the second part of this work 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, inelastic 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. A very efficient procedure for carrying out the simulations is developed.;The above mentioned model is tested 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. The model subsequently is applied to the accommodation of H2, N2, CO and CO2 on surface materials employed on vehicles in low Earth orbit. Finally, the model is used to simulate experimental results for N2 scattering from disk-drive air bearings and from the early earth mineral pyrite. The model is capable of accurately reproducing the results of experimental measurements on these highly-corrugated surfaces, as well as offering deeper insight of how corrugation affects scattering and trapping processes.