The influence of the N(2D) and N(2P) states in the ionization of the pink afterglow of the nitrogen flowing DC discharge

Abstract

In the present work, we extensively discuss the role of N(2D) and N(2P) atoms in the ionization processes of pink afterglow based on optical emission spectroscopy analysis and kinetic numerical modelling. We studied the pink afterglow generated by a nitrogen DC discharge operating at 0.6 Slm−1 flow rate, 45 mA discharge current and pressures ranging from 250 to 1050 Pa. The 391.4 nm nitrogen band was monitored along the afterglow furnishing the relative density of the N2+(B2Σ+u, v = 0) state. A numerical model developed to calculate the nitrogen species densities in the afterglow fits the excited ion density profiles well for the experimental conditions. From the modelling results, we determine the densities of the N+, N2+, N3+, and N4+ ions; the calculations show that the N3+ ion density predominates in the afterglow at the typical residence times of the pink afterglow. This behaviour has been observed experimentally and reported in the literature. Furthermore, we calculate the fractional contribution in the ionization for several physical-chemical mechanisms in the post-discharge. Even with the N3+ ion density being dominant in the afterglow, we find through the calculations that the ionization is dominated by the reactions N(2D) + N(2P) → N2+(X2Σ+g) + e and N2(a′1Σ−u) + N2(X 1Σg+, v > 24) → N4+ + e. The ion conversion mechanisms, or ion transfer reactions, which are responsible for the fact that the N3+ density dominates in the post-discharge, are investigated.

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.…”