N2 dissociation and kinetics of N(4S) atoms in nitrogen DC glow discharge

Abstract

N2 dissociation in pure nitrogen plasma has a long history of research. It seems to be a complex process which comprises many reactions involving various electronic and vibrational nitrogen states whose contributions can vary depending on conditions. In this paper, we studied N2 dissociation in the stationary N2 discharge both experimentally and theoretically. We used a DC glow discharge in a quartz tube in pure N2 at moderate pressures (5–50 Torr). The degree of dissociation, atomic nitrogen loss rate and gas temperature were measured by applying optical emission spectroscopy (OES) and as a result an ‘effective’ rate constant for nitrogen dissociation was obtained across a wide range of the reduced field E/N. The analysis of N2 dissociation was carried out using a specially developed 1D radial self-consistent model which takes into account the spatial inhomogeneities of species concentrations, E/N, electron energy distribution function, Tgas etc, together with fairly complete plasma-chemical kinetics and all the cross-sections known to date for electron kinetics. The model was successfully validated through the experimental results obtained for electric field, gas temperature and N atom density. Comprehensive analysis of closely coupled processes in nitrogen plasmas—gas heating, VDF formation and N2 dissociation—was carried out. Simulations reproduced the experimental data on well and allowed us to evaluate the different contributions of the various dissociation channels considered. It was shown that the nitrogen dissociation mechanism in the stationary N2 discharge is provided by direct electron impact via the excitation of the pre-dissociative states from the vibrationally excited nitrogen molecules N2(X, υ). The upper limit for the rate constant of the processes N2(A) + N2(14 ⩽ υ ⩽ 19) → N + N + N2 was estimated to be 5 · 10−14 cm3 s−1.