Gas-surface scattering with multiple collisions in the physisorption potential well

The problem of gas-surface collisions is developed in terms of a theoretical formalism that allows calculations of multiple collisions of particles trapped in the physisorption well. The calculation uses classical dynamics for describing the scattering. Multiple collisions in the well are treated with an iterative process that can be followed to very high order numbers until all trapped particles are desorbed. Example calculations are carried out for scattering in a one-dimensional potential. Two different initial gas distributions are considered, a monoenergetic incident beam and an equilibrium gas appropriate for calculating the energy accommodation coefficient. Each part of the scattered distribution, the directly scattered fraction, the desorbed fraction and the trapped fraction, can be examined separately. The desorbed distribution is often found to deviate significantly from an equilibrium state. This behavior is contrary to long-standing hypotheses in which it is assumed that the trapped-desorbed fraction escapes the surface in an equilibrium distribution at the surface temperature. The energy distribution of the trapped fraction is examined and from it desorption times are calculated. The formalism is used to calculate the equilibrium energy accommodation coefficient, including contributions of desorbing particles. It is found that the effective energy accommodation coefficient of that fraction of the incident beam that is initially trapped and then subsequently desorbed may be less than unity.