Kinetic study of pulsed desorption flows into vacuum

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Abstract
The one-dimensional flow of particles thermally desorbed from a plane surface into vacuum is studied on the basis of a Monte Carlo simulation of the Boltzmann equation. It is assumed that particles desorb during a finite period of time with a fixed temperature. It is shown then that the flow is determined by a single parameter, which is essentially the number of monolayers Θ desorbed, and is inverse to the Knudsen number of the problem. The cases of negligible (Θ≪1) and in part also of very intense (Θ≫1) desorption fluxes can be treated analytically using collision-free flow and ideal gas dynamics, respectively. Here the simulation yields very good agreement with theory, indicating that the code can be used on a wide range of Knudsen numbers. For the gas-dynamical case, a Knudsen layer is formed by the equilibration of the flow in the vicinity of the surface; it is studied in some detail. Simulation of flows with Θ≲1 shows that the particle distribution is far from thermal equilibrium everywhere in the flow, and deviates strongly from the analytically accessible cases. For stronger desorption fluxes, the flow is partially equilibrated, such that the velocity component in the direction of the flow reaches thermal equilibrium, although with a smaller temperature than the velocity component perpendicular to the flow. The time integrated velocity spectra of particles measured far away from the surface are discussed for moderate desorption fluxes. They are surprisingly well described by a thermal distribution, even though no Knudsen layer forms in this case.