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

Abstract

The evolution of individual v = 0–10 vibrational levels of N2(A3) 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) 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, 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), N2 and N2 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 Δv = 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.