Spectral analysis of the light emitted by streamers in hydrocarbon liquids

This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geometry. Experimental spectra of the light emitted by bush-like and filamentary streamers are composed of the H/sub /spl alpha//-Balmer line, the C/sub 2/ Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C/sub 2/ in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of C/sub 2/ and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited C/sub 2/ do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of H/sub /spl alpha// are in the range 4/spl times/10/sup 16/ to 7/spl times/10/sup 16/ cm/sup -3/ for filamentary streamers and 2 to 6/spl times/10/sup 17/ cm/sup -3/ during the breakdown phase. For slow bush-like streamers, the electron density is not measurable.