Study of temporal dissociation and atomic excitation rates in a flowing N₂ DC discharge and post-discharge

Abstract

In this work, mechanisms of molecular dissociation and atomic excitation occurring in a flowing nitrogen DC discharge and its post-discharge were studied experimentally and theoretically. A specific discharge experimental condition for the pink afterglow plasma occurrence in the post-discharge tube is analyzed. We employed the Optical Emission Spectroscopy (OES) and Langmuir probes to measure the reduced electric field (E/N), electron density (n ),e and the gas and N (X2 1Σ+g) vibrational temperatures in the discharge positive column. OES was also employed in the post-discharge for measurements of relative densities of N(4S) and N(2D) atoms in the pink afterglow. Two well established kinetic numerical models, one for the positive column, and the other one for the post-discharge, were utilized to calculate the molecular dissociation and atomic excitation rates as functions of the gas residence time in the positive column and also in the nitrogen post-discharge. We analyzed the role of 13 molecular dissociation mechanisms, and seven atomic excitation mechanisms in the positive column and pink afterglow. Results demonstrate that the positive column dissociation processes are dominated by direct electron impact mechanism in the earlier discharge gas residence times, and that for longer times reactions between electronically excited states and N2(X1Σ+g, v) vibrational states become the dominant dissociation mechanisms. It is also observed that dissociation processes occurring in the pink afterglow present relevant rates as compared to the same processes occurring in the positive column, demonstrating the high effectiveness of such processes in the post-discharge. The N(2D) and N(2P) excitation mechanisms were also examined. We observed that molecular dissociation and atomic excitation mechanisms strongly depends on the N2(X1Σ+g) vibrational distribution function of the discharge and post-discharge.