Electric field determination from intensity ratio of and N2 bands: nonequilibrium transient discharges in pure nitrogen

Abstract

We developed an extension of the spectrometric method to estimate a reduced electric field (E/N), which is applicable in nitrogen-containing plasmas. The method is based on the intensity ratio of the emission bands of the first negative system (FNS) of and the second positive system (SPS) of N2. It uses the emission occurring in the wavelength interval 375–410 nm, which includes six SPS and two FNS bands. The choice of the spectral window is guided by much simpler acquisition and processing of experimental data than the SPS(0, 0) and FNS(0, 0) pair that is typically used. Following this idea, we construct a kinetic model for pure molecular nitrogen, which determines the population of the upper states responsible for the FNS and SPS emission. Moreover, we perform sensitivity analysis of the kinetic model, which allows us to reveal the most significant processes for the investigated intensity ratios. For these processes, we provide an in-depth review of the kinetic data that are available in the literature. We use the fact that the spectral window investigated contains bands to obtain three independent intensity ratios with sufficient signal-to-noise ratio ((FNS(0, 0)/SPS(0, 2), FNS(0, 0)/SPS(1, 4), FNS(0, 0)/SPS(2, 5)), which are usable for more accurate electric field determination. We also provide analytical formulas representing intensity ratio dependencies on E/N. Furthermore, we focus on different spectrometric representations of FNS and SPS bands, which also affect the precision of E/N determination. We examine the FNS/SPS band profiles in terms of different rotational temperatures and instrumental functions. Finally, we propose a simple procedure that enables the use of bandhead intensities in the intensity ratio dependencies, thus avoiding the need to evaluate integral band intensities from the recorded spectra.