Monitoring the removal of excited particles in He/Ar/H2low temperature afterglow plasma at 80–300 K

Abstract

Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H 3 + ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He 2 excited dimers ( a 3 Ʃ u + ) in plasmas in pure helium and in helium with small admixture of Ar and H 2 . By monitoring the plasma decay and its dependence on [H 2 ] and [Ar] in the afterglow in pure He and in He/Ar/H 2 mixture we estimated the rate of plasma thermalization in the temperature range of 80–300 K. The inferred deexcitation rate coefficients for reaction of helium metastable atoms with H 2 were (0.9 ± 0.3) × 10 −10 cm 3 s −1 , (1.9 ± 0.2) × 10 −10 cm 3 s −1 and (2.7 ± 0.2) × 10 −10 cm 3 s −1 at 80 K, 140 K and 300 K, respectively. The effective recombination rate coefficients for H 3 + were evaluated from the decay of the electron number density. We propose the lower estimate for the saturation of the effective recombination rate coefficient in H 3 + and D 3 + dominated plasma.