LIF study of N2(A3?u, v = 0-10) vibrational kinetics under nitrogen streamer conditions

The evolution of individual v = 0-10 vibrational levels of N2(A3$\Sigma_{\text{u}}^{+}$ ) metastable species produced by filamentary streamer discharge was investigated by the laser-induced fluorescence technique. Triggered single streamer filament was periodically produced in pure nitrogen at a pressure of 200 torr and metastable species were monitored during the streamer channel decay in the centre of the discharge gap. The observed dynamics of N2(A3$\Sigma_{\text{u}}^{+}$ ) vibrational levels follow two very different scenarios: while higher (v > 6) vibronic levels decay exponentially in hundreds of nanoseconds, the populations of lower levels (v <= 6) definitely increase, first reaching a local maximum on a microsecond timescale and then decreasing afterwards. Population maxima of N2(A3$\Sigma_{\text{u}}^{+}$ , v <= 6) levels occur after the streamer onset with a certain delay, which decreases with increasing vibrational number. Interpretation of experimental observation based on a 0D kinetic model of the post-discharge period takes into account the most important processes redistributing populations between the N2(A3$\Sigma_{\text{u}}^{+}$ ), N2 $\left({{\text{B}}^{3}}{{\Pi}_{\text{g}}}\right)$ and N2$\left({{\text{C}}^{3}}{{\Pi}_{\text{u}}}\right)$ vibronic levels. The model reproduces experimental observations fairly well, including observed maxima delays occurring due to the collisional cascade, which transfers metastable species from higher even/odd vibrational levels towards v = 0/v = 1 terminal levels through the ?v = 2 vibrational relaxation mechanism. A calibration procedure based on the rate of energy-pooling processes was used to determine absolute populations of the v = 0 and 1 levels from LIF data, and the model results were utilized to place on an absolute scale all the higher (v > 1) measured vibronic levels. Vibrational distributions obtained from calibrated LIF data at selected instants show a reasonable qualitative agreement with model predictions. Population maxima exceeding 3 × 1014 cm-3 were fixed for v = 2 and 3 vibrational levels, while the lowest v = 0 level reaches only 8-9 × 1013 cm-3. Lastly, we show that the observed rate of the v = 2 level decay is not compatible with published rate constants for the v = 2 -> v = 0 vibrational relaxation.