Transformation of nitrogen SPS spectra emitted from streamer discharge head within a sub-nanosecond — Nanosecond range

The paper presents some results on spectroscopic studies of the filamentary streamer discharge in short air gap in stage of primary streamer propagation. Basically, we have found that the mid-resolved (with spectral resolution around 0.2-0.3 nm) nitrogen second positive system (SPS) spectra emitted from the primary streamer head changes essentially in form within some nanoseconds. Namely, main peak near the band head formed by the P-branches of Pi3-to-Pi3 transition turns almost into a widened twin-peak hump, relative intensities of each sub-peaks change in time resulting finally in a quite usual one-peak form; with inessential modification of short-wave part of the SPS-band as whole formed mainly by the R-branches. In attempt to give an appropriate realistic interpretation of this phenomenon we have analyzed different possible instrumental factors as well as mechanisms applied in streamer theory and molecular spectroscopy, such as: 1) dynamic breaking of the spin-axis coupling of the Pi3-states resulting in an appearance of Pi3(a)-to-Pi3(b)- and Pi3(b)-to-Pi3(b)-transitions additionally to a standard Pi3(a)-to-Pi3(a)-transition; 2) modeling of luminosity emitted from the actual high-gradient too narrow streamer head propagating with very high speed; 3) fast eventual heating of neutral gas within streamer head; 4) possible non-even illumination of the monochromator entrance slit due to very strong gradient of all physical parameters within streamer head and finally as well as actual sub-nanosecond temporal resolution of the measuring system; 5) Zeeman and Stark-effects. Stark-effect and instrumental peculiarities are supposed to be most adequate reasons for the phenomenon, which after development of relevant detailed theory might be applicable to determine/estimate electric field and its spatial gradient, respectively