Quantitative characterization of a highly effective atomic hydrogen doser

Abstract

An atomic hydrogen doser of the Bertel type was characterized in terms of the degree of dissociation and angular distribution of the effusing particles. In this doser hydrogen is dissociated in a tungsten tube which is heated by electron bombardment. Various experimental techniques were used to determine the degree of dissociation as function of temperature and gas flux. It is shown that simple equilibrium considerations cannot be applied to obtain the degree of dissociation accurately. Nevertheless, for sufficiently small gas flux and temperatures above 1850 K, the degree of dissociation approaches 100%. The angular distribution was determined by a gold foil on a goniometer as detector, which is sensitive to atomic hydrogen only. The experimental results were compared with Monte Carlo simulations. A strongly forward focused distribution is observed which allows efficient atomic hydrogen dosing. This doser was used to measure absolute initial sticking coefficients for atomic hydrogen on various single crystal metal surfaces: S0(H)=1.0 on Ni(111), 0.9 on Ni(110), 0.7 on Al(111), 0.6 on Al(100) and 0.5 on a polycrystalline gold foil, respectively.