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

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Abstract
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 broader than cosine. In addition, at the lowest temperature employed (∼85 K), the velocity distributions of the desorbing atoms are well described by Maxwell–Boltzmann distributions characteristic of the surface temperature. At higher temperatures, these distributions are still approximately Boltzmann, but the characteristic temperature falls below Ts. The ‘‘lag’’ between this effective temperature and Ts increases with Ts and is most pronounced for atoms desorbing at angles close to the normal. We show that the desorption results are very close to the predictions of a model in which angular and velocity distributions for desorption are synthesized by applying detailed balance arguments to the trapping data. Similarly the trapping results are close to trapping curves extracted from the desorption data.