H₃⁺: The Simplest Polyatomic Molecule in the Laboratory and the Interstellar Medium

Ben McCall

University of Illinois

The molecular ion H₃⁺ consists of just three protons, bound together by two electrons. As a charged and purely hydrogenic species, it plays a key role in initiating the network of ion-molecule chemistry that is responsible for the formation of a wide variety of interstellar molecules. This talk will review the astrochemistry of H₃⁺, the detection of this ion in dense and diffuse molecular clouds, and two surprises that have emerged. The first surprise was that the column density of H₃⁺ in diffuse molecular clouds is roughly equal to that in dense clouds. Storage ring measurements of the rate of electron recombination of H₃⁺ have since led to the realization that the cosmic-ray ionization rate in diffuse clouds must be about an order of magnitude larger than in dense clouds, and a theoretical treatment of cosmic-ray ionization has suggested that this may be due to a previously unrecognized large flux of low-energy (2-10 MeV) cosmic rays. The second surprise was that the observed ortho:para ratio of H₃⁺ was not consistent with thermal equilibrium. Recent experiments in our lab, together with a simple analytical chemical model, have suggested that the ortho:para ratio is determined by the steady state of the proton-swapping chemical reaction H₃⁺ + H₂ → H₃⁺ + H₂ (the most common bimolecular reaction in the universe!).