Interaction of Hydrogen with Tungsten

Measurements have been made under ultra‐high vacuum conditions of the rates and isotherms for the adsorption of hydrogen on tungsten, the rate of desorption of hydrogen from tungsten, and the rate of formation of atomic hydrogen by an incandescent tungsten filament. When hydrogen chemisorbs on tungsten at 77°, the sticking probability is 0.1 and two distinct states of binding, α and β, may be isolated at adsorption pressures as low as 2.5×10^—9 mm. Analysis of desorption kinetics of the most tightly bound state, β, shows that hydrogen chemisorbs as atoms since evaporation is second order in adsorbed hydrogen. For n_β< 30×10¹² molecules/cm², $v_M{\text{=(5×10}}^{\text{−3}}{\mathrm{)n}}^2\exp \text{(−31\hspace{0.17em}000/RT)}$ molecules/cm²‐sec. At higher coverages, a decrease in heat of desorption is observed which is due to surface heterogeneity. At equilibrium, the amount adsorbed in the β state obeys a Temkin isotherm between 273° and 373°K; evaporation of hydrogen in the β state occurs down to temperatures as low as 77°K. Isosteric heats agree with heats derived from desorption measurements. The α state, populated before the β state is saturated at 77°K, is proportional to hydrogen pressure and probably consists of molecular hydrogen on top of the primary atomically chemisorbed layer. The rate of evaporation of atomic hydrogen when the filament temperature is raised above 1100°K is $v_A{\text{=(2.2×10}}^{13}\mathrm{)n}\exp \text{(−67\hspace{0.17em}000/RT)}$ molecules/cm²‐sec. For pressures below 10^—7 mm and filament temperatures above 1475°K, the fraction of molecules striking the filament surface that is dissociated is constant and at least 0.05. Values of surface coverage and the rate of formation of atomic hydrogen as a function of filament temperature and hydrogen pressure are derived from experimental data.