Spectroscopic investigations of N₂(A³ Σ_u⁺) metastables in the spatial ionisation growth in nitrogen

The authors describe a spectroscopic technique to investigate the transport processes of electrons and the quenching processes of excited metastable molecules under the condition of non-self-sustained Townsend discharges. The technique is applied to pure N₂, and N₂ including NO as an impurity at E/N=282.5 Td and currents less than 10^-9 A. The quenching factor of N₂(A³ Sigma _u⁺) metastable by NO, defined by (1-(density of N₂(A³ Sigma _u⁺) in N₂ with NO)/(density of N ₂(A³ Sigma _u⁺) in pure N ₂)), is measured as a function of total gas density from the emission spectrum both of the second positive bands of N₂ and of the gamma system of NO. The spatial profile of N₂(A³ Sigma _u⁺) is also monitored and estimated by the emission of the gamma system. It will be noted that half of the N₂(A³ Sigma _u⁺) metastables are deactivated with only 20 PPM of NO at N=1.94*10¹⁷ cm^-3 in the low pressure ionisation growth chamber. The results give a quantitative explanation for the rapid decrease of the secondary ionisation coefficient in N₂ in the presence of small amounts of the impurity. Also the radiative lifetime and the self-quenching rate coefficient are estimated in the present experimental conditions.