The local dissociation phenomenon in a nitrogen afterglow

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 . Its rate constant was estimated, being approximately 5 × 10−12 cm3 s−1.

Neutral and Excited Molecules and Atoms Densities in the Positive Column of Flowing N2 DC Discharges

“…The singlet species account for vibrational states as source generation mechanisms. This VDF heating effect is the same one that occurs in the generation of the pink afterglow phenomenon in the nitrogen flowing post-discharge [9][10][11][12][13][14][15][16].…”

“…, p is the gas pressure in Torr) [9]. This procedure to obtain the electron density was utilized in several of our works on nitrogen discharges and post-discharges [8,[10][11][12][13][14][15][16][17][18]. We utilized high purity N 2 gas (99.999%) in this study.…”

Study of ionization and ion transfer rates in flowing N2 DC discharges and post-discharges

“…The pure nitrogen flowing post-discharges were also intensely studied in the last decades [6][7][8][9][10][11][12][13][14][15][16]. These studies concern the pink afterglow or short-lived afterglow phenomenon.…”

“…These studies concern the pink afterglow or short-lived afterglow phenomenon. The pink afterglow phenomenon is a nitrogen plasma occurring in the post-discharge region, where reionization and re-excitation of species occur for some experimental conditions of gas flow rate, discharge power (SW or RF discharge [6][7][8]11]), or discharge current (DC discharge [9,10,[12][13][14][15][16]), and gas pressure. The interesting fact in this phenomenon is that ionization and excitation of species occur outside the external zone of the electric field influence of the discharge, in the post-discharge region.…”

“…The re-ionization and re-excitation of nitrogen species are driven from the flow of vibrational quanta through the N 2 (X 1 Σ + g , v) vibrational states via vibrational-vibrational (V-V) exchange reactions between these molecules. The vibrational quanta flux populates the intermediate and high N 2 (X 1 Σ + g , v) vibrational states which become the precursors of excitation [12,14], dissociation [13], and ionization [9,15,16] physical-chemical processes in the post-discharge, sustaining the plasma.…”