Gas temperature determination from rotational lines in non-equilibrium plasmas

A review

Peter J Bruggeman, N. Sadeghi, D. C. Schram, V. Linss

Research output: Contribution to journalReview article

170 Citations (Scopus)

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.

Original languageEnglish (US)
Article number023001
JournalPlasma Sources Science and Technology
Volume23
Issue number2
DOIs
StatePublished - Jan 1 2014

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nonequilibrium plasmas
gas temperature
molecular spectra
rotational spectra
diatomic molecules
deduction
recommendations
budgets
availability
degrees of freedom
spectrometers
excitation

Keywords

  • gas temperature
  • optical emission spectroscopy
  • rotational temperature

Cite this

Gas temperature determination from rotational lines in non-equilibrium plasmas : A review. / Bruggeman, Peter J; Sadeghi, N.; Schram, D. C.; Linss, V.

In: Plasma Sources Science and Technology, Vol. 23, No. 2, 023001, 01.01.2014.

Research output: Contribution to journalReview article

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N2 - 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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