The Velocity Distribution of Hydrogen Molecules Desorbed from Metal Surfaces

An essential step in many chemical reactions occurring at gas-solid interfaces is the associative desorption reaction, A(ad) + B(ad) AB(g), where two atoms adsorbed on the surface combine to form a molecule that desorbs to the gas phase. This reaction has been studied in several laboratories1 by a variety of techniques because of its importance to heterogeneous catalysis, high-temperature oxidation, chemical vapor deposition, and the degassing of metals. Existing data are not sufficiently detailed, however, to provide rigorous tests of theoretical models of the kinetics and energetics of associative desorption. We believe that more rigorous tests will be possible with the unique data obtained by the experimental technique described herein. It has been generally assumed that the velocity distribution of molecules desorbed from a solid surface corresponds to that of molecules effusing through an orifice from an equilibrium gas phase at a temperature equal to that of the solid. This assumption leads to the expectations: (i) that the spatial distribution of the desorbed molecules will be directly proportional to cos 0, where 0 is the angle of inspection measured from the surface normal; (ii)that the speed distribution at all values of 0 will be of the Maxwellian form with temperature equal to that of the solid. Recent results obtained independently 23 in two different laboratories show, however, that the spatial distribution of hydrogen desorbed from nickel deviates markedly from the cos 0 relation. More specifically, these data indicate that the fraction of the molecules desorbing in the general direction of the surface normal is much greater than that corresponding to the effusion of an equilibrium gas through an orifice. Van Willigen Z has reported similar data for the