The Influence of Metastable Species and Rotational Quantum Numbers on the Derivation of OH (A–X), NO-γ (A–X) and N₂ (C–B) Bands Rotational Temperatures in an Argon Gas-Liquid-Phase Plasma Discharge

In this work, the rotational temperatures of the atmospheric pressure AC-excited argon gas-liquid-phase discharge estimated by employing disparate spectroscopic methods on the rotational structure of molecular OH (A–X), NO- $\gamma $ (A–X), and N₂ (C–B) bands-spectra emanating from the gas-phase discharge region were compared with those obtained from the liquid-phase region to analyze their variations based on the metastable species and rotational quantum numbers. The electrical characteristics and images of the discharge were also examined to analyze the discharge nature. Realization of the estimated values for rotational temperatures, as an estimation of the gas temperature, depend on thermalization of the accounted excited rotational population levels. Excitations to higher rotational quantum numbers ( ${N}'$ ) inhibited thermalization of the rotational distributions, and the rotational temperatures were found to increase proportionally with excitations to higher ${N}'$ numbers. The phenomenon of rotational excitation to higher ${N}'$ numbers was particularly prominent for the wet discharge region, and thus, substantially higher rotational temperatures were obtained. The excitation to higher ${N}'$ numbers is related to the basic production processes largely those involving $\textrm {Ar}_{\textrm {meta}}^{\ast } $ and N₂ (A) metastable states that populate predominantly the levels with large ${N}'$ numbers (e.g., ${N}'=8$ and ${N}'=25$ , respectively for OH and NO), and thus, their contemplated rotational population is not indicative of the kinetic gas temperature. With respect to spectral diagnostics techniques, the Boltzmann plot constructed by adopting a unique single fitting model (compared to the conventional double fitting models approach used for liquid discharges) only calculating the rotational population of the $Q_{1}$ branch of OH (A–X) for lower ${N}'$ numbers ( ${N}'\le 4$ ) affords the opportunity to estimate the gas temperature in wet plasma discharge.