Direct current N 2 flowing discharges were generated and conditions for the pink afterglow were obtained. Emissions from N 2 (B, C) and N + 2 (B) radiative states were studied as a function of pressure, flow rate and post-discharge position. A one-dimensional kinetic model accounting for N 2 (X, v), N 2 (A, B, C, a, a , a ), N( 4 S), N + 1−4 (X) and N + 2 (B) species has been developed in order to describe the experimental observations. The analysis on the complete set of processes assumed in this paper has provided possible generation mechanisms for N atoms, N + 2 (B) ions and N 2 (B, C) electronically excited states as well as for metastable ones. It has been shown that ionization, excitation and dissociation processes occur simultaneously at the post-discharge region when the vibrational distribution function of N 2 (X, v) states heats as resulting from the efficient V-V pumping mechanism, which is very sensitive to pressure conditions. Here, the pink afterglow is described as a non-equilibrium plasma, i.e. ambipolar diffusion for ions and the condition of charge neutrality are assumed.

Nitrogen flowing DC discharges were generated between two side-armed electrodes in a drift tube. The discharges operated at gas residence times (t) of ∼4 × 10−4 s, reduced electric fields (E/N) between 90 and 118 Td, and electron densities (ne) between 1010 and 1011 cm−3. A kinetic numerical model was elaborated to study the discharge kinetics. The model calculates the densities of 18 electronic states of nitrogen in the discharge, including the 45 vibrational levels of the N2(X1Σ+g) molecules, as functions of the gas residence time. The model is employed to describe the density profiles of neutral and excited atomic and molecular species, and nitrogen ions, along with the N2(X1Σ+g) vibrational distributions for our experimental conditions. The N2(X1Σ+g) vibrational and gas temperatures, E/N, ne, and the N2(B3Πg), N2(C3Πu), and N2+(B2Σ+u) relative densities were measured in the discharge by optical emission spectroscopy and double probes. The experimental determined gas temperature (Tg), electron density, and reduced electric field were used in the calculations of the electron energy distribution function and reaction rate constants. The vibrational temperature (Tv) and excited species densities measured were compared to the calculated values from the model. Although much attention has been devoted to the study of nitrogen DC discharges in the last few years, this work presents for the first time the N+ – N4+ and N2+(B2Σ+u) ion density distribution together with the densities of 13 atomic and molecular nitrogen states as functions of the discharge gas residence time and N2(X1Σ+g) vibrational distributions calculated for experimental conditions of low pressure DC discharges operating at short residence times.

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The post-discharge generated by a direct current N2 flowing discharge was studied by optical emission spectroscopy. The experimental conditions were such that the short-lived afterglow could be detected. The post-glow emissions, first positive and 1st negative N2 systems, were recorded along the post-discharge tube up to times of the order of 0.1 s (late afterglow). The experimental parameters, gas and vibrational temperatures, were measured and utilized in a numerical kinetic model developed for calculations of the N(4S) absolute density along the post-discharge. Moreover, the model was employed in estimations of the percentage of each excitation channel to the excitation of the N2(B 3Πg, v = 11) state as a function of post-discharge time. Among them, the main excitation channels are the N(4S) three-body recombination and the pooling with the states. From the computation of the percentage of contribution of the three-body recombination mechanism in the overall excitation of the N2(B 3Πg, v = 11) state, the first positive emission intensity at 580.4 nm wavelength was corrected. After correction, one expects that the emission intensity would be proportional to the square root of the N(4S) density. Therefore, [N(4S)] experimental estimations can be achieved as a function of post-discharge position or post-discharge time. The experimental profiles are in good agreement with our previous theoretical results and the spatial parametrization to the 580.4 nm band provides a significant advance to the experimental method developed by Bockel et al (1995 Surf. Coat. Technol. 74–75 474).

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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.

We have generated a N 2 flowing discharge sustained by surface waves employing a 2.45 GHz high frequency source. The N 2 dissociation was studied in the discharge and post-discharge regions by optical emission spectroscopy (OES) as a function of the experimental parameters: discharge power (30-160 W) and absolute pressure (1-20 Torr), at 0.5 Slm −1 flow rate. The N(2p 3 4 S 0 ) absolute density was measured in the discharge by actinometry. We have introduced the effect of the N 2 (X 1 + g ) vibrational temperature in the actinometry equation. Such a consideration was made based on the work of Catherinot and Sy (1979 Phys. Rev. A 20 1511) which presents a profound discussion about the quenching mechanism of the N(3p 4 S 0 ) by N 2 (X 1 + g , v) states. The 811.5 nm Ar and 821.6 nm N lines, from 2p 9 → 1s 5 and 3p 4 P 0 → 3s 4 P transitions have been utilized here. Both are easily observed in the surface wave discharge. Further, the NO chemical titration was carried out in the late post-discharge furnishing the N(2p 3 4 S 0 ) absolute density in that region. The extinction point of the 580.4 nm band of the N 2 1st positive system was measured. The density values obtained by actinometry are validated by comparison with those measured in the post-discharge region. The two sets of data, function of pressure and discharge power, may be related by the mass conservation equation. The work provides the experimental N(2p 3 4 S 0 ) absolute density values as a function of power and pressure only by application of OES techniques. Moreover, we demonstrate here that the N(3p 4 P 0 ) state is quenched by N 2 (X 1 + g , v 2) molecules forming N 2 (b 1 u , v = 0) and N( 4 S) states, in agreement with Catherinot and Sy.