Kinetics of N-2(A (3)Sigma(+)(u)) molecules and ionization mechanisms in the afterglow of a flowing N-2 microwave discharge

To gain an understanding of the processes responsible for the formation of the well-known short-lived afterglow (SLA) or pink afterglow of nitrogen, d ifferent diagnostic techniques are implemented in the afterglow of a 440 Pa microwave nitrogen discharge in a 3.8 cm diameter flow tube. Using the int racavity laser absorption spectroscopy technique, we measure the space-depe ndent absolute density of N-2(A (3) Sigma (+)(u); upsilon = 0) metastable m olecules, as well as their rotational temperature, which in fact correspond s to the gas temperature, T-g. The density of N-2 (A (3)Sigma (+)(u)) molec ules is about 5 x 10(17) M-3 at the end of the discharge zone (z = 4 cm). I t then continuously decays by almost two orders of magnitude to reach a min imum around z = 12 cm before monotonically increasing to a secondary maximu m of 5 x 10(16) M-3 located around z = 19 cm. It then slowly decays at long er distances. The space-dependent N-2(B (3)Pi (g)) fluorescence intensity e volves 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 ca rried to this region by the gas flow. Contributions to the creation of N-2( A (3) Sigma (+)(u)) molecules from N-N atom recombination, as well as from high U vibrational levels of the ground-state N-2(X, upsilon) molecules are analysed. To account for the high density of N-2 (A (3) Sigma (+)(u)) mole cules, despite the (2-3) U x 10(21) M-3 density of N atoms in the afterglow , we propose that N(P-2) metastable atoms, resulting from the efficient que nching of N-2(A (3) Sigma (+)(u)) U molecules by N(S-4) atoms, react immedi ately with vibrationally excited N-2(X (1) Sigma (+)(g); upsilon " greater than or equal to 10) molecules to again form N-2(A (3) Sigma (+)(u)) molecu les. The absolute electron density in the SLA was also measured by microwav e interferometry. Its axial dependence also shows a pronounced minimum at z = 12 cm before reaching a maximum of 6 x 10(15) M-3 at z = 19 cm. The poss ible processes for this local ionization, in the absence of any electric fi eld, could be binary collisions of the electronically excited molecules, an d/or of the vibrationally excited ground-state molecules in upsilon " great er than or equal to 30 levels.