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Vibrational excitation potentially enhances the energy efficiency of plasma dissociation of stable molecules and may open new routes for energy storage and process electrification. Electron, vibrational and rotational temperatures were measured by in situ Thomson and Raman scattering in order to assess the opportunities and limitations of the essential vibration-translation non-equilibria in N2, CO2 and CH4 plasma. Electron temperatures of 1.1–2.8 eV were measured in N2 and CH4. These are used to confirm predominant energy transfer to vibrations after an initial phase of significant electronic excitation and ionization. The vibrational temperatures initially exceed rotational temperatures by almost 8000 K in N2, by 900 K in CO2, and by 300 K in CH4. Equilibration is observed at the 0.1 ms timescale. Based on the vibrational temperatures, the vibrational loss rates for different channels are estimated. In N2, vibrational quenching via N atoms is identified as the dominant equilibration mechanism. Atomic nitrogen population reaches a mole fraction of more than 1%, as inferred from the afterglow emission decay, and explains a gas heating rate of 25 K μs−1. CH4 equilibration at 1200 K is predominantly caused by vibrational-translational relaxation in CH4–CH4 collisions. As for CO2, vibrational-translational relaxation via parent molecules is responsible for a large fraction of the observed heating, whereas product-mediated VT relaxation is not significantly contributing. It is suggested that electronic excitation, followed by dissociation or quenching contributes to the remaining heat generation. In conclusion, the time window to profit from vibrational excitation under the present conditions is limiting practical application.

published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User

published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User

Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from ~7 mm diameter at 100 mbar to ~2 mm at 400 mbar. We find that, simultaneously, the initial non-equilibrium between ~2 eV electron and ~0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modelling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors.