Kinetics of Desorption of the β-Nitrogen States Chemisorbed on Tungsten

Abstract

The kinetics of desorption of the strongly chemisorbed β1 and β2 nitrogen states on tungsten have been studied. Independent techniques of isotopic mixing and flash-filament desorption have been employed. The rate of production of 14N15N (g) from 14N2(g) and 15N2(g) in a steady-state flow experiment is completely accounted for by the kinetically determined rate of desorption from the chemisorbed layer. β1-nitrogen appears to desorb by first-order kinetics; β2-nitrogen desorbs in a complex manner with higher-order kinetics. The apparent β2 desorption order decreases markedly as temperature increases. A superposition of two or more unresolved substates in the β2 state is postulated. The temperature variation of desorption order can be accounted for by a lack of achievement of equilibrium between the spatially separate β2 substates during desorption at scan rates of 12.5°K/sec or greater. Above 1500°K, where the steady-state coverage of nitrogen is very low, a sticking coefficient of less than 0.02 has been measured. The sticking coefficient is nearly constant in the range 1500°—1700°K and over a 25-fold pressure range.