Two helium-neon laser interferometers were used to obtain the electron and neutral-atom densities in an afterglow plasma. The interferometric technique utilized allows one to obtain both the spatial and temporal dependence of the electron decay. The two gases studied were helium and argon at 2-8 Torr and 0.3-0.8 Torr, respectively. The electron density was in the range of 2×10¹³<Nₑ<10¹⁵ cm^-3 and the electron temperature in the range 1000<Tₑ<7000^∘K. The electron temperature was measured by comparing the relative atomic line intensities and by inference from the recombination coefficient. The electronic recombination in helium, argon, and helium-argon mixtures was found to be consistent with the predictions of Bates, Kingston, and McWhirter for collisional-radiative recombination. The electron temperature inferred from the measured recombination coefficient indicates a pronounced electron temperature gradient across the tube which is believed to be due to electron heating effects in the afterglow.
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Gusinow et al. (1966) studied this question.