Recombination of H+3 ions with electrons in He/H2 ambient gas at temperatures from 240 K to 340 K

Abstract

We have studied the recombination of ions with electrons in a stationary afterglow experiment either in pure hydrogen or in mixtures of helium, hydrogen, and argon. The decay of the ion density during the afterglow was monitored using near-infrared cavity ring-down absorption spectroscopy. The gas temperatures ranged from 240 K to 340 K. We find that three-body recombination, where molecular hydrogen acts as a third body, increases the recombination rate significantly. However, assisted three-body recombination saturates at hydrogen densities above and the recombination rate then becomes nearly independent of the neutral gas density. The saturation can lead to the erroneous conclusion that the recombination is purely binary and this appears to be the cause of some inconsistencies between previously reported recombination rate coefficients. At temperatures 240–340 K the obtained three-body recombination rate coefficient is independent on temperature () and it is larger by five orders of magnitude than those expected from the classical theory for atomic ions of Bates and Khare (). The observed dependences on density suggest that a substantial fraction of neutral complexes formed in capture of electrons by ions do not rapidly predissociate but can be stabilized by interactions with third bodies. Saturation occurs if the capture step is rate limiting rather than stabilization.