A comprehensive multi-species comparison of rotational temperature probes in a DC Ar/N 2 micro-hollow cathode discharge

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Abstract Accurate gas temperature ( T Gas ) determination in microplasmas is critical for optimizing their applications, yet isolated diagnostic approaches may yield misleading results, especially under strong non-equilibrium conditions. Here, high resolution rotational spectra of N 2 (C), OH(A), NH(A) and NO(A), generated in the plasma jet of a DC Ar/N 2 microhollow cathode discharge (MHCD), are recorded and their associated rotational temperatures ( T rot ) are cross compared. A detailed experimental analysis and robust fitting of the rotational spectra are performed, achieving a reliable estimation of T Gas . The dominant formation mechanisms of these species and their corresponding impact on rotational population distributions are also interrogated. Particularly, our findings indicate that the T rot of N 2 (C) is significantly influenced by energy transfers from argon metastables (Ar m ) and spectral interference from NH(A). This makes it unreliable as a thermometric probe in Ar-rich MHCD, unless complex analyses are employed. In contrast, OH(A) rotational population distribution appears to be less sensitive to Ar-induced perturbations across various discharge currents and pressures, providing more straightforward results. For all molecules considered, this study reveals the MHCD conditions under which all the measured T rot can be reliably considered to be in equilibrium with T Gas . This highlights the importance of cross-validating multiple thermometric probes and investigating relevant excitation kinetics when measuring T rot in reactive microplasmas.

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