Abstract
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.
Highlights
Nonequilibrium plasma is known to be a useful tool for many applications
Discharges ignited in molecular gases can often be described by three distinct temperatures: (i) electronic temperature, characterizing the portion of electronically excited molecules, (ii) vibrational temperature (Tvib), representing the distribution of the population among vibrational levels, and (iii) rotational temperature (Trot), which is related to the distribution of the population among rotational levels
We describe in detail how to fit spectra of molecules with non-Boltzmann distributions which can be used for cases of overlapping molecular spectra
Summary
Nonequilibrium plasma is known to be a useful tool for many applications. The reason for its broad range of employment is the ability to affect chemical bonds and surface properties without the usage of high temperature. In order to obtain detailed maps of rotational and vibrational temperatures in a relatively well-thermalized microwave discharge, none of the abovementioned packages has been found suitable as they do not support fast batch processing of spectroscopic data For this reason, a software package named massiveOES has been developed, focusing on the fast evaluation of Boltzmann-distributed spectra. The package has been extended by a tool for state-by-state fitting, motivated by the necessity to evaluate spectra with the non-Boltzmann distribution. massiveOES has been made open-source, written in the free Python language.41–44 This way, its functionality is not inherently limited to the purpose it was originally designed for, but the software can be freely adjusted to solve other spectroscopic problems.
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