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Ben McCall - University of Illinois

Wednesday, May 27 – 9:45-10:30AM


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


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. Laboratory experiments 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!).


Related Reference(s):

N. Indriolo , B. D. Fields, and B. J. McCall, "The Implications of a High Cosmic-Ray Ionization Rate in Diffuse Interstellar Clouds," Astrophysical Journal, 694, 257-267 (2009).

B. A. Tom, V. Zhaunerchyk, M. B. Wiczer, A. A. Mills, K. N. Crabtree, M. Kaminska, W. D. Geppert, M. Hamberg, M. af Ugglas, E. Vigren, W. J. van der Zande, M. Larsson, R. D. Thomas, and B. J. McCall, "Dissociative Recombination of Highly Enriched Para-H3+," Journal of Chemical Physics, 130, 031101 (2009).

M. Larsson, B. J. McCall and A. E. Orel, "The Dissociative Recombination of H3+ -- A Saga Coming to an End?" Chemical Physics Letters, 462, 145-151 (2008).

N. Indriolo, T. R. Geballe, T. Oka and B. J. McCall, "H3+ in Diffuse Interstellar Clouds: A Tracer for the Cosmic-Ray Ionization Rate," Astrophysical Journal, 671, 1736–1747 (2007).

T. P. Snow and B. J. McCall, "Diffuse Atomic and Molecular Clouds," Annual Review of Astronomy and Astrophysics, 44, 367-414 (2006).