Self-consistent kinetic model of low-pressure - flowing discharges: II. Surface processes and densities of N, H, species

This work is the second of two companion papers devoted to the kinetic modelling of low-pressure DC flowing discharges in mixtures. While the first paper was mainly concerned with bulk discharge processes, the present one investigates surface processes involving dissociated N and H atoms, which are essential to understand the discharge properties. The kinetic model for surface processes developed here takes into account: (a) physical adsorption and desorption of N and H atoms; (b) chemical adsorption and desorption of both types of atoms at vacant chemically active sites on the surface; (c) surface diffusion of physisorbed and atoms; (d) the reactions of chemisorbed and atoms with gas phase N and H atoms and with physisorbed and atoms, leading to the formation of gas phase , and chemisorbed (NH) molecules. The latter molecules can either be desorbed or react with H, , to produce gas phase NH, and molecules. The probability of wall losses for N, H, NH, and the rates for wall production of NH, , have been obtained for Pyrex glass as a function of the wall temperature and the relative concentrations of N, H, NH, , . A number of important parameters for surface processes have been estimated from detailed analysis of experimental data. Measurements of relative changes in N, H and concentrations in discharges as a function of the percentage have been interpreted in terms of the model.