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Description
In this work, the non-equilibrium nature of argon-based radiofrequency (RF) and microwave plasmas at atmospheric pressure is evidenced. In particular, rotational temperature ($T_{rot}$) and neural gas temperature ($T_{g}$) are found to be unequal in every condition tested, even though they are often assumed equal in such plasmas in the literature. Such a finding was made possible through the use of a hyperfine spectrometer developed and commercialized by LightMachinery Inc. Offering a 2 pm resolution over a simultaneous range of 25 nm, it was tuned to the 820-845 nm range in which the broadening of the Ar $2p_{2}-1s_{2}$ and Ar $2p_{3}-1s_{2}$ (Pashen notation) transitions is strongly affected by the neutral gas temperature. Therefore, for these emission lines, the experimental broadening was much smaller than the other broadening sources, ensuring a precise and accurate determination of $T_{g}$.
The microwave plasmas were produced inside a fused silica tube using a surfaguide-type wave launcher, while the RF plasmas were produced inside a fused silica tube placed between two electrodes. In both cases, an admixture of $H_{2}O$ or $N_{2}$ were added to the argon flow in order to observe either the OH ($A^{2}Σ^{+}- X^{2}Π_{i}$) or the $N_{2}^{+}$ ($B_{2}Σ_{u}^{+}- X_{2}Σ_{g}^{+}$) molecular systems. A rotational temperature was then calculated using the Boltzmann plot method. $T_{rot}$ and $T_{g}$ values were obtained every centimeter along the plasma columns. For the microwave plasma with an admixture of $H_{2}O$, $T_{g}$ values of over 2000 K were obtained while $T_{rot}$ values were in the 1400 K range. For the microwave plasma with an admixture of $N_{2}$, $T_{rot}$ values were found to rise to ~3200 K whereas $T_{g}$ values only increased to ~2400 K. The same discrepancies were found in the much colder RF plasmas ($T_{g}$~400 K while $T_{rot}$~515 K). Therefore, since the rotational temperatures did not equal the gas temperature in every condition tested, it is concluded that the rotational-translational equilibrium cannot not be assumed for RF and microwave argon-based plasmas at atmospheric pressure.
