Chemistry in low-pressure cold plasmas: ions of astrophysical interest

The ionic chemistry of various hydrogen mixtures (H-2/N-2, H-2/O-2, and H-2/air) has been studied in low-pressure hollow cathode discharges. The major ions identified in the different discharges (H-3(+), N2H+, H3O+ and NH4+) have been also found in astronomical observations or predicted in astrochemical models. The relative stability of the protonated ions in the various mixtures has been investigated in detail. In discharges of H-2 with small amounts of N-2, O-2 and air, appreciable amounts of NH3 and H2O were formed at the reactor walls. The preponderance of the protonated ions in these plasmas was found to be largely dictated by the proton affinity of their respective molecular precursors. Even for small amounts of water and ammonia, proton transfer reactions tend to concentrate the positive charge in H3O+ and, especially, in NH4+ ions. These results support the predictions of some astrochemical models indicating that these ions could be dominant in warm astronomical environments where H2O and NH3 molecules evaporate from dust grain mantels.