Nuclear Spin Dependence in Reactions of H3 in the Laboratory and the Interstellar Medium By

Hydrogen is the most abundant element in the universe. For this reason, the chemistry of the interstellar medium is largely dominated by the interaction of hydrogenic neutrals and ions. The hydrogen molecule, H2, and the simplest polyatomic ion, H3 , each exist in one of two forms identified as ortho or para, which are defined by their total nuclear spins. These spin modifications have different spectral signatures which can be observed both in the laboratory and the interstellar medium. H2 and H3 have been used to probe the conditions of interstellar clouds, but there are gaps in our understanding of the mechanisms by which the para-H3 fraction is enriched in diffuse clouds and para-H2 is enriched in dense molecular clouds. Dissociative recombination is the primary destruction mechanism for H3 in diffuse clouds. The rate coefficient for this process has been measured for highly enriched para-H3 and compared with the rate coefficients for less para-enriched plasmas. The results show that dissociative recombination for para-H3 occurs at a faster rate than for ortho-H + 3 , which validates recent theoretical predictions but does not explain the enrichment of para-H3 in the diffuse interstellar medium. The H3 + H2 reaction, which is arguably the most common bimolecular reaction in the universe, is explored as the possible mechanism by which the enrichment of both species can occur. Two models that we developed from recent theoretical work are used to predict the behavior of the reaction under low temperature conditions. Laboratory measurements were taken over a range from 300 K down to 80 K, and demonstrate that the branching ratio of proton hop to hydrogen exchange is temperature dependent. This is the first study of this reaction at the cold astrophysical temperatures where it occurs most of the time. A comparison of the experimental results with observational data provides strong evidence that this reaction is the driving mechanism for the steady state enrichment of para-H3 in diffuse clouds, and predictions from the models imply that it is also responsible for the enrichment of para-H3 and para-H2 in dense clouds. The results of this work have implications for the use of the H3 spin modifications as astrophysical probes, for understanding the deuterium chemistry of the interstellar medium, and for the fundamental chemical physics of these very simple molecules.