The local dissociation phenomenon in a nitrogen afterglow

Abstract

Abstract We used the optical emission spectroscopy diagnostic to study the nitrogen afterglow of a pure N2 flowing dc discharge operating under particular experimental conditions to facilitate the simultaneous occurrence of the pink afterglow (PA) and the Lewis–Rayleigh afterglow. The PA is a special kind of nitrogen plasma occurring outside the direct influence of an external electric field. The phenomenon results from the flux of energy, introduced in the nitrogen molecules by the electrons in the discharge region, from the lower to the higher vibrational levels due to vibrational–vibrational (V–V) and vibrational–translational (V–T) exchange reactions. We studied the following set of experimental conditions: discharge electric current (I = 15–50 mA), gas pressure (p = 200–1070 Pa) and gas flow rate (Q = 400–1000 sccm). The emissions of the first positive system of the nitrogen molecules were monitored from the end of the discharge down to the end of the post-discharge tube. A kinetic numerical model developed to investigate the nitrogen afterglow generated a calibrating factor for the 580.4 nm band in such a way that the relative density of the N(4S) atoms could be measured along the afterglow. The experimental results indicated that N(4S) atoms are created locally in the afterglow producing atomic density profiles that follow the behaviour of the other species studied experimentally in the PA, such as , N2(B 3Πg), N2(C 3Πu), , , N+, , , N(2D) and N(2P). The numerical model was also used to fit the N2(B 3Πg), and the N(4S) experimental density profiles and to evaluate the participation of several kinetic pathways capable of producing local dissociation in the N2 afterglow. It was found that the dominant dissociation channel in the PA is the reaction 4)\to {\rm N}(^4{\rm S})+{\rm N}(^4{\rm S})+{\rm N}_2 (X\,^1\Sigma _{\rm g}^+ )$ ?> . Its rate constant was estimated, being approximately 5 × 10−12 cm3 s−1.