Formation of , v = 0–3) metastable species in decaying nitrogen streamer

Abstract

Evolution of , v = 0−3) metastable species produced by triggered single filamentary streamer discharge was investigated utilizing emission spectrometry and laser-induced fluorescence techniques. Triggered streamers were produced repeatedly in point-to-plane DBD electrode geometry in pure nitrogen at 200 Torr and metastable species were monitored in the centre of the discharge gap. Populations of all four monitored vibronic levels increase during streamer channel decay phase, reaching local maximum in a microsecond timescale and decreasing afterwards. Maximum population of an individual , v) vibronic level occurs with a certain delay after streamer onset τd(v), which decreases with increasing vibrational number. The longest delay τd = 5 ± 0.5 µs was fixed for the lowest metastable level v = 0, whereas τd values of 4.4 ± 0.5, 1.75 ± 0.25 and 0.85 ± 0.25 µs were fixed for v = 1, 2 and 3, respectively. Coupled evolution of the two lowest , v = 0, 1) levels was monitored independently by applying an indirect method based on emission intensity of the N2 Herman infrared system. Very good agreement between emission and fluorescence based diagnostics was achieved for later post-discharge times t > τd. A simple 0-D model of , v) post-discharge kinetics based on vibrational relaxation and electronic quenching of individual vibronic levels reproduces experimental observations fairly well. According to the model, observed maxima likely occur due to the collisional cascade that transfers metastable species from higher even/odd vibrational levels towards v = 0/v = 1 terminal levels through the Δv = 2 vibrational relaxation mechanism.