Abstract
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.
Highlights
Over the last several decades, non-equilibrium discharges on and within liquid surfaces have rapidly evolved because of their great impact in diverse fields such as cancer treatment, lithotripsy, plant growth enhancement, water decontamination, chemical analysis, and nanoparticle synthesis [1-8]
Optical emission spectroscopy has long been accepted as a valuable technique for characterizing and exploring the basic physical parameters of plasmas and is frequently applied to predict the rotational temperature (Trot) of plasmas based on measurement of the excited rotational population distributions of various diatomic molecules such as OH (A–X), NO-γ (A–X), and N2 (C–B)
It is important to have an in-depth understanding of this particular characterization technique, especially for the case of atmospheric pressure non-equilibrium plasmas generated in a gas-liquid environment or in a complex mixture comprising various molecular species
Summary
Over the last several decades, non-equilibrium discharges on and within liquid surfaces have rapidly evolved because of their great impact in diverse fields such as cancer treatment, lithotripsy, plant growth enhancement, water decontamination, chemical analysis, and nanoparticle synthesis [1-8]. In comparison to conventional atmospheric pressure glow discharges excited between metallic electrodes, the chemistry and physics related to plasma discharges in liquids electrodes are somewhat less well understood. This is because these discharges are typically excited in both the gas- and liquid-phases, and are relatively chaotic in nature [9]. In the discharge assembly where the liquid typically serves as one of the electrodes, it is found that the electrodes tend to evaporate and undergo deformation These factors add significant complexity to the experimental configuration, and the associated discharge processes become fairly intricate when compared with the non-thermal gas-phase discharges (plasmas) between metallic electrodes. Non-Boltzmann distributions which are witnessed in the rotational spectra of non-equilibrium plasmas obviously demonstrate the significance of being able to inspect carefully the rotational structure of the spectra
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