Photoionization in developing low temperature plasma streamer discharges in air

Abstract

Experimental investigation of the vacuum and extreme ultraviolet emission spectra of gas discharges (i.e. low temperature plasmas) in the developmental phase was completed. These experiments confirm the often conjectured conclusion that molecular nitrogen transitions of sufficiently short wavelength are responsible for photoionization of molecular oxygen [1]. By varying the gas used in the experiments, Trienekens et al. were able to show that sub-100 nm emission predominantly originates from molecular nitrogen [2]. More refined experiments in air revealed transitions from the c4'1Σ +u excited state of N2 to be the primary contributor to sub-100 nm emission. This directly contradicts the common assumption that transitions from the b1nu and b'1Σ +u excited states of N2 (i.e. Birge-Hopfield transitions) are the primary contributors to photoionization in air. The efficacy of this conclusion was further benchmarked using a theoretical model of plasma development where it was observed that transitions from the c4'1Σ +u excited state of N2 did in fact provide sufficient emission to support positive streamer formation.