Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

Abstract

The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literature. It is found, that during the discharge the measured vibrational excitation agrees well with predictions obtained from the rates for resonant electron collisions calculated by Laporta et al (2014 Plasma Sources Sci. Technol. 23 065002). The predictions are based on the electric field during the discharge, measured by electric field induced second harmonic generation Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), Lepikhin et al (2020 J. Phys. D: Appl. Phys. 54 055201) and the electron density, which is deduced from the field and mobility data calculated with Bolsig+ Hagelaar and Pitchford (2005 Plasma Sources Sci. Technol. 14 722–33). In the afterglow a simple kinetic simulation for the vibrational subsystem of nitrogen is performed and it is found, that the populations of vibrational excited states develop according to vibrational-vibrational transfer on timescales of a few microseconds, while the development on timescales of some hundred microseconds is determined by the losses at the walls. No significant influence of electronically excited states on the populations of the vibrational states visible in the CARS measurements ( v ≲ 7 ) was observed.

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: I. Measurement procedure and results

“…As the electric field is the same in all three cases, the difference in slope should be mainly caused by a higher electron density in the 4 kV case. A more detailed discussion of the slope, Ṙh , is given in the companion paper [1]. The fact, that one of the measurements is performed with a longer pulse duration, is also reflected in figure 13 by a longer rise time of R h (note the two vertical lines at about 200 ns and 250 ns marking the corresponding ends of the pulses).…”

“…Firstly, at these time only states up to v = 3 can be detected as the densities of the other states are below the detection limit. Secondly, (25) is proposed to analyze the characteristics of the newly excited molecules (for further details the reader is referred to the companion paper [1]). Before the discharge pulse there are no "newly" excited molecules yet and the non-zero value of the fitting parameter R h only accounts for the small deviation from a normal Boltzmann distribution remaining from the previous voltage pulse still visible in the excited states v = 1 − 3 and T vib,h suffers a large uncertainty, as it strongly depends on the higher states, which are not detected.…”

“…So, to be able to use the two-temperature distribution in its original intent the newly excited molecules need to overtake the remaining deviation from a Boltzmann distribution. The criterion used here and in [1] is that R h needs to at least surpass two times its initial value from before the discharge, so that the majority of the molecules belonging to the hot distribution consists of molecules excited during the current discharge pulse. The data points which are excluded from further discussion by the mentioned criterion are marked by a gray background in figure 14.…”

“…This is rooted in the interpretation that the hot distribution is made up by molecules which are excited during the current discharge cycle. The further motivation of this two-temperature distribution function (TTDF) is discussed in more detail in the companion paper [1] and here only the formula is given:…”

“…This allows to capture the more complex vibrational dynamics in that time period. The measurement results are discussed in more detail and compared to simple models in a companion paper [1].…”