Adsorption and desorption kinetics with no precursor trapping: Hydrogen and deuterium on W {100}

Abstract

Using a molecular beam technique, accurate dissociative adsorption sticking probabilities are reported for both H 2 and D 2 interacting with W{100}, over a wide range of surface and beam temperatures, and of surface coverage. By following the scattering of HD and D 2 from the crystal surface produced from mixed beams of H 2 and D 2, isothermal desorption data have also been obtained at crystal temperatures between 380 and 480 K which yield model-independent variations of desorption activation energy and desorption pre-exponential factors with surface coverage. Within an experimental accuracy of 5%, no isotope effect was observed in adsorption or desorption kinetics for H 2 and D 2. The sticking probability s falls linearly with coverage, according to the expression s = (0.72 − 5.4 × 10 −4{T} B ) (1 − θ 2 ) where T B is the gas temperature; s is independent of substrate temperature over the range 200 to 1150 K. The data indicate no influence of trapping into a precursor state. At low coverages the desorption energy E d is 159 kJ mol −1, falling precipitately at θ = 0.46 to 88 kJ mol −1. At the same coverage, the desorption pre-exponential term v falls by a factor of 10 7. The results are discussed in terms of the known adsorbate-induced, coverage-dependent displacive phase transition which occurs for this system. It is proposed that at low coverages desorption occurs with a normal pre-exponential (of 7 × 10 15 s −1) from the localised pinched dimer structure; above θ = 0.5, however, desorption occurs from a delocalised ad-layer, with a low pre-exponential factor (~ 4 × 10 8 s −1). Since the adsorbate is localised at all coverages at 300 K, it is further proposed that a localised-delocalised phase transition, with Δ S ≈ 60 J mol −1 K −1 and Δ H ≈ 21 kJ mol −1, occurs between 300 K and the desorption temperature.