Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

Abstract

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.

Translational, rotational, vibrational and electron temperatures of a gliding arc discharge

“…The translational temperature of the gliding arc discharge is usually obtained from modeling [39][40][41] or estimated to be equal to the rotational temperature based on the assumption of a fast rotational-translational relaxation [25]. However, as Bruggeman and associates pointed out [42], the rotational-translational equilibrium is not always valid in non-thermal plasmas, and determination of the translational temperature from emission spectra should be made with a caution. The translational temperature can be directly measured using laser-induced Rayleigh scattering technique [43].…”

Investigation of hydrocarbon oil transformation by gliding arc discharge: comparison of batch and recirculated configurations

“…This is probably due to the direct reaction of plasma with dodecane which causes increased degradation and less vaporisation than in the batch liquid treatment. Most commonly in N 2 discharges, the dominant emission comes from the second positive N 2 C → B in the UV region [23,24]. This is not observed in the humid nitrogen GAD generated in this case.…”

Determination of vibrational and rotational temperatures in highly constricted nitrogen plasmas by fitting the second positive system of N2 molecules

“…33 The dependence of neutral gas temperature on input power can be understood by taking the basic dissipation power into consideration. 24 The input power W 0 is principally dissipated in the four channels, 24 the dissociation of neutrals W 1 , the ion creation/loss to the chamber wall W 2 , the conduction of thermal energy from the plasma to the chamber wall W 3 , and the convection of thermal energy by flow from the discharge tube W 4 .…”

“…According to the discussion on the possible broadening mechanisms in Ref. 33, pressure broadening is only significant at low temperature and high pressures, thus the temperature dependences of the FWHM induced by the van der Waals and resonance broadening were not taken into account. For fitting ro-vibrational spectra, an experimental method has been proposed to determine the Voigt profile parameters by measuring the shape of an atomic line, such as Ar I line at 794.8 nm and Hg I line at 435.8 nm.…”