Stationary afterglow (SA) experiments with cavity ring down absorption spectrometer (cw-CRDS) have been used to study recombination of H3+ ions with electrons. To characterize the plasma during the afterglow we monitored the time evolution of density of He2 excited dimers (a3Ʃu+) in plasmas in pure helium and in helium with small admixture of Ar and H2. By monitoring the plasma decay and its dependence on [H2] and [Ar] in the afterglow in pure He and in He/Ar/H2 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 H2 were (0.9 ± 0.3) × 10−10 cm3 s−1, (1.9 ± 0.2) × 10−10 cm3 s−1 and (2.7 ± 0.2) × 10−10 cm3 s−1 at 80 K, 140 K and 300 K, respectively. The effective recombination rate coefficients for H3+ 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 H3+ and D3+ dominated plasma
