The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

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We investigated the rotational excitation of the nitrogen molecule ion in a pulsed magnetron sputter discharge (Mg target, pressure 0.1–2.0 Pa) and a 150 Pa dc glow discharge in dependence on various process parameters. For this purpose we used optical emission spectroscopy of the 0–0 band of the first negative system of the (FNS0–0) and calculated the rotational temperature by fitting the spectra. Often, the best fit could be achieved assuming two populations of the molecules having two different rotational temperatures. These temperatures and their contributions to the spectrum of the FNS0–0 show a significant dependence on the process parameters. The lower temperature is in the range of 370–800 K and is believed to be equal to the translational temperature of the neutral gas. The higher temperature is in the order of 1500–3000 K and its origin is most probably the excitation of the state by heavy particle impact connected with rotational excitation.

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