Evolution of N₂(A³${\Sigma }_u^{+}$) in streamer discharges: influence of oxygen admixtures on formation of low vibrational levels

The formation of N₂(A³ ${\Sigma }_u^{+}$) metastable species, produced by cathode-directed streamer discharge, was investigated using the technique of laser-induced fluorescence. A triggered single streamer filament was periodically produced in pure nitrogen (and in nitrogen with admixtures of oxygen) at total pressure of 50 Torr and metastable species were monitored during the streamer channel decay in the centre of the discharge gap.

We revealed the dynamics of individual vibrational (v  =  0–8) levels of N₂(A³ ${\Sigma }_u^{+}$) for various oxygen admixtures (0–20%). In pure nitrogen, the observed evolution of the N₂(A³ ${\Sigma }_u^{+}$) during the decaying streamer channel is evidence of initial vibrational relaxation of high vibrational levels towards the v  =  2 and 3 levels, followed by a delayed increase of terminal (v  =  0, 1) levels. A calibration procedure based on the rate of energy-pooling processes was used to place all detected vibronic levels in pure nitrogen on the absolute scale. Population maxima exceeding 1  ×  10¹⁴ cm⁻³ were fixed for the v  =  2 and 3 vibrational levels, while the lowest v  =  0 level reaches only 3  ×  10¹³ cm⁻³.

Populations of v  =  2–5 vibrational levels were also estimated for N₂  +  O₂ mixtures after scaling of laser-induced fluorescence signals obtained at various oxygen admixtures. The total N₂(A³ ${\Sigma }_u^{+}$) population in an air-like mixture is formed mainly by v  =  3–4 vibronic levels with the population maximum of ∼3  ×  10¹³ cm⁻³ fixed at the shortest analyzed delay. This observation, together with the fact that we were unable to detect v  =  0 and 1 levels (fluorescence signals below detection threshold), gives a strong evidence of the inhibition of Δv  =  2 vibrational relaxation towards terminal v  =  0 and 1 levels, causing much lower populations of the lowest v  =  0–1 levels.

By analyzing data obtained in pure nitrogen and in nitrogen with three different oxygen admixtures, we have estimated the quenching rate constants of N₂(A³ ${\Sigma }_u^{+}$, v)  +  O₂ and N₂(A³ ${\Sigma }_u^{+}$, v)  +  N₂ processes for v  =  2–6 vibrational levels which are consistent with data from the literature. Quenching rate constants for nitrogen reflect the N₂(A³ ${\Sigma }_u^{+}$, v)  →  N₂(A³ ${\Sigma }_u^{+}$, v  −  2) relaxation process, however the v  =  2 rate constant shows a much larger value compared with the published data. (paper)