Evolution of N(4S) atoms produced under nitrogen streamer conditions: time-resolved TALIF study at reduced pressures

Abstract

The evolution of N(4S) species produced by filamentary streamer discharge was investigated by two-photon absorption laser-induced fluorescence technique. A triggered single streamer filament was periodically produced in pure nitrogen at pressures of 20, 50, 100 and 200 Torr, and N(4S) species were monitored in the center of the discharge gap during the streamer formation, decay and afterglow. Under all of the investigated pressures, the population maxima of nitrogen atoms were observed in a microsecond timescale; i.e. well after the extinction of the streamer event. The concentration of nitrogen atoms increased with pressure. The maximum N(4S) concentration measured at 20 Torr was about 1013 cm−3, while it exceeded 1015 cm−3 at 200 Torr. After reaching maxima, atomic densities exhibit hyperbolic decay, which was followed up to the post-discharge time of 0.5 ms. Our results show that only a small part of N(4S) is produced during the streamer transit or within 200 ns after the transit (20%–25% increase with respect to the background density), while most of the nitrogen N(4S) atoms are produced during the streamer channel decay in timescales from units to several tens of microseconds.