Gas-surface interactions of atomic nitrogen with vitreous carbon

Abstract

We have conducted beam-surface scattering experiments with the intent of identifying the reaction mechanisms that are relevant to the ablation of carbon by N atoms at surface temperatures relevant to hypersonic flight. A pulsed molecular beam containing N and N2 with translational energies of 460 and 808 kJ mol-1, respectively, was directed at a vitreous carbon surface held at temperatures in the range 1023–1923 K. Time-of-flight distributions were collected for scattered products with fixed incidence and final angles of θi = θf = 45°. Inelastically scattered N and N2 and reactively scattered CN were the only products observed. The scattering dynamics of N and N2 were independent of surface temperature and were indicative of purely impulsive scattering. The scattering dynamics of CN suggested that this reaction product is formed by a mechanism that occurs in thermal equilibrium with the surface – i.e., a Langmuir-Hinshelwood mechanism. The reaction probability to produce CN has an Arrhenius temperature dependence with an activation energy of 207 kJ mol-1. The relevance of the current results to the hypersonic ablation of carbon in the presence of dissociated air is discussed.