Kinetic description of non-Boltzmann OH rotational distribution in nonequilibrium stationary plasmas

Statistical physics of two-temperature rotational energy distributions in stationary plasmas

Two-temperature rotational energy distributions from rarefied diatomic molecules are very often observed in laboratory plasmas. There has been much debate over the years about the physical meaning of this kind of rotational energy distributions and the associated statistical physics. We show here that under certain reasonable assumptions and constraints the condition of Shannon-Jaynes entropy maximization may produce a two-temperature distribution. This may happen, for instance, when a system is simultaneously coupled to different thermal baths. In plasmas this is possible because rarefied molecular species may be immersed in a medium where electrons and the dominant atomic species are quasidecoupled, each of them acting as distinct thermal baths. Considering that molecular species may interact both with electrons and heavy neutral species, we may ask what should be the resulting molecular energy distribution. We answer this question in this paper and give some examples on how this can be used to interpret experimental molecular distribution from partially ionized plasmas.

Deducing rotational quantum-state distributions from overlapping molecular spectra

A novel method for fast and robust calculation of Boltzmann plots from molecular spectra is presented. Its use is demonstrated on the OH(A-X) spectrum near 310 nm. A limitation of the method is identified: for overlapping spectra of the OH(A-X) and N₂(C-B, Δv = 1) band sequence, the calculation may often fail due to insufficient number of measured points. This is solved by introducing experimentally determined bounds for the N₂(C) rotational distribution. Three cases are presented: (i) with undisturbed OH(A-X) emission, (ii) with strong emission of N₂(C-B) in the said spectral range, and (iii) with weak but not negligible nitrogen emission. In case (ii), the data in the spectral range 306-320 nm are sufficient for the analysis. In case (iii), information from another spectral range with undisturbed N₂(C-B) emission is necessary. These illustrate all relevant cases often encountered in laboratory plasmas. The calculated Boltzmann plots are not further analyzed in this article but can be used for development and validation of kinetic models with rotational resolution. The implementation of the reported method using the massiveOES software package written in the Python language is available in the supplementary material.