Kinetics of N₂(A³Σ_u⁺) molecules and ionization mechanisms in the afterglow of a flowing N₂microwave discharge

Abstract

To gain an understanding of the processes responsible for the formation of the well-known short-lived afterglow (SLA) or pink afterglow of nitrogen, different diagnostic techniques are implemented in the afterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameter flow tube. Using the intracavity laser absorption spectroscopy technique, we measure the space-dependent absolute density of N2(A 3Σu+; v = 0) metastable molecules, as well as their rotational temperature, which in fact corresponds to the gas temperature, Tg. The density of N2(A 3Σu+) molecules is about 5×1017 m-3 at the end of the discharge zone (z = 4 cm). It then continuously decays by almost two orders of magnitude to reach a minimum around z = 12 cm before monotonically increasing to a secondary maximum of 5×1016 m-3 located around z = 19 cm. It then slowly decays at longer distances. The space-dependent N2(B 3Πg) fluorescence intensity evolves in exactly the same way. A simple kinetic model is developed and we conclude that metastable molecules are locally formed in the SLA and not carried to this region by the gas flow. Contributions to the creation of N2(A 3Σu+) molecules from N-N atom recombination, as well as from high vibrational levels of the ground-state N2(X, v) molecules are analysed. To account for the high density of N2(A 3Σu+) molecules, despite the (2-3) ×1021 m-3 density of N atoms in the afterglow, we propose that N(2P) metastable atoms, resulting from the efficient quenching of N2(A 3Σu+) molecules by N(4S) atoms, react immediately with vibrationally excited N2(X 1Σg+; v''⩾10) molecules to again form N2(A 3Σu+) molecules. The absolute electron density in the SLA was also measured by microwave interferometry. Its axial dependence also shows a pronounced minimum at z = 12 cm before reaching a maximum of 6×1015 m-3 at z = 19 cm. The possible processes for this local ionization, in the absence of any electric field, could be binary collisions of the electronically excited molecules, and/or of the vibrationally excited ground-state molecules in v''⩾30 levels.