Dynamical parameters of desorbing molecules

Molecular beam techniques have been used to probe the dynamics of the trapping and trapping–desorption of Ar at a hydrogen‐saturated W(100) surface. Trapping probabilities have been measured as a function of incidence energy Ei, and angle θi for a surface temperature Ts of 85 K. We find that this probability scales approximately with Ei  cos θi, rather than Ei or the so‐called ‘‘normal energy’’ Ei  cos2 θi. Trapping probabilities approach unity for low energies, falling to 0.5 and 0.05 for Ei  cos θi ∼30 and 100 meV, respectively. The time‐of‐flight distributions of scattered Ar are clearly bimodal in many cases, having both direct–inelastic and trapping–desorption components. The latter component has been characterized over a wide range of conditions to provide information on the desorption of Ar from this surface. We find that desorbing species emerge with a near‐cosine angular distribution for Ts ≂85 K. However, these distributions become increasingly noncosine as Ts is raised, becoming substantially b...

Desorption and trapping of argon at a 2H–W(100) surface and a test of the applicability of detailed balance to a nonequilibrium system

The problem of thermal transpiration between parallel plates and in a cylindrical capillary is considered by using the Maxwellian diffuse‐specular reflection at the surface. The relevant integral equations are solved by a simple numerical scheme and results for both the Poiseuille flow and the thermal creep flow rate are obtained. Some recently reported variational results for the Poiseuille flow appear to be inaccurate (off by about 10%–15%), and there is a need for further refinement in the variational calculations. The effect of partial accommodation is found more significant in the free molecule region than in the near continuum region. Thus to minimize the surface effects, the experiments on the thermal conductivity measurements via thermal transpiration would best be conducted in the near continuum region.

Ivan Kuščher

Papers

Reciprocity in scattering of gas molecules by surfaces