Influence of oxygen traces on pure nitrogen post discharge kinetics

Summary form only given. Although the discharges and post-discharges in nitrogen and its mixtures with oxygen are subjects of many studies during last more than 50 years, there are only a few studies of pure nitrogen containing oxygen in trace concentrations under 1 %. The aim of our work is the extension of our recent studies carried out in DC post-discharge created in Pyrex tube using pure nitrogen containing oxygen traces in the low concentration range from 0 up to 2000 ppm. In presented work, the decaying plasma up to 50 ms was studied in quartz tube (i.d. of 12 mm) by the emission spectroscopy in the spectral range of 200 - 1050 nm. Three nitrogen spectral systems and NO^beta and NO^gamma bands were identified in the investigated spectral region. The total gas pressure was 1000 Pa and the discharge current was kept at 200 mA. The relative populations of N₂ (B ³Pi_g) and N₂ (C ³Pi_U) and N₂ ⁺ (B ²Sigma_u ⁺) states were calculated in the dependence on post- discharge time and on the oxygen concentration. The so-called nitrogen pink afterglow effect was well visible for all levels of all three nitrogen electronic states but its character depended on the electronic state. The lower levels of N₂ (B ³Pi_g) up to v = 9 showed relatively sharp pink afterglow between 5 and 15 ms that increased with the oxygen concentration increase up to 700 ppm, higher oxygen concentration started up its quenching. The higher levels of the N₂ (B ³Pi_g) state (including also levels higher than predissociation limit at v=12) as well as N₂ (C ³Pi_u) state also showed the increase of the pink afterglow intensity but the population decrease beyond pink afterglow maximum was significantly slower. The population dependence on oxygen concentration was nearly the s- - ame as was described above. The molecular ionic state N₂ ⁺ (B ²Sigma_u ⁺) showed the pink afterglow effect similar to that for lower N₂ (B ³Pi_g) levels but there was a strong quenching of its populations by oxygen at concentrations higher than 700 ppm and the effect completely disappeared at oxygen concentrations above 1500 ppm. Both NO states showed significant populations at the lowest vibrational level, only. The NO populations increased with the increase of the decay time and they were directly proportional to the oxygen concentration. A very simplified kinetic explanation of the observed phenomena was done by the quenching of highly excited nitrogen ground state molecules by NO (NO dissociation and its immediate formation in B state) and by the energy transfer between nitrogen and NO species.