Kinetics of N2(A 3Σu+) molecules and ionization mechanisms in the afterglow of a flowing N2 microwave discharge

Abstract

To gain an understanding of the processes responsible for theformation of the well-known short-lived afterglow (SLA) or pink afterglowof nitrogen, different diagnostic techniques are implemented in theafterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameterflow 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 rotationaltemperature, which in fact corresponds to the gas temperature, Tg. The density of N2(A 3Σu+) molecules is about5×1017 m-3 at the end of the discharge zone (z = 4 cm). Itthen continuously decays by almost two orders of magnitude to reach aminimum around z = 12 cm before monotonically increasing to a secondarymaximum of 5×1016 m-3 located around z = 19 cm. It thenslowly decays at longer distances. The space-dependentN2(B 3Πg) fluorescence intensity evolves in exactly thesame way. A simple kinetic model is developed and we conclude thatmetastable molecules are locally formed in the SLA and not carried to thisregion by the gas flow. Contributions to the creation ofN2(A 3Σu+) molecules from N-N atom recombination, aswell as from high vibrational levels of the ground-state N2(X, v)molecules are analysed. To account for the high density ofN2(A 3Σu+) molecules, despite the (2-3)×1021 m-3 density of N atoms in the afterglow, we proposethat N(2P) metastable atoms, resulting from the efficient quenching ofN2(A 3Σu+) molecules by N(4S) atoms, reactimmediately with vibrationally excited N2(X 1Σg+;v''⩾10) molecules to again form N2(A 3Σu+) molecules. The absolute electron density in the SLA was alsomeasured by microwave interferometry. Its axial dependence also shows apronounced minimum at z = 12 cm before reaching a maximum of6×1015 m-3 at z = 19 cm. The possible processes for thislocal ionization, in the absence of any electric field, could be binarycollisions of the electronically excited molecules, and/or of thevibrationally excited ground-state molecules in v''⩾30levels.