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Merged beam studies for astrochemistry
  Molecular Physics seminar

Monday 17 November 2014
from 10:00 to 11:00
at FA31
Speaker : Nathalie de Ruette (Atomic Physics)
Abstract : The chain of chemical reactions leading towards life is thought to begin in molecular clouds when atomic carbon and oxygen are "fixed" into molecules. Reactions of neutral atomic C with H⁺₃ is one of the first steps in the gas phase chemistry leading to the formation of complex organic molecules within such clouds [1]. Water, believed to be essential for life, can form via a chain of gas-phase reactions that begin with neutral atomic O reacting with H⁺₃. Uncertainties in the thermal rate coefficient for each of these reactions hinder our ability to assess the validity of the present chemical network leading to the synthesis of complex organic molecules. Theory and experiment have yet to converge in either the magnitude or temperature dependence for each of these reactions. Theory provides little insight as fully quantum mechanical calculations for reactions involving four or more atoms are too complex given current capabilities. On the other hand, measurements of cross sections and rate coefficients for reactions of atoms with molecular ions are extremely challenging. This is due to the difficulty in producing sufficiently intense and well characterized beams of neutral atoms. Ion trap studies of C on cold H⁺₃ were performed at 1000 K, much hotter than molecular clouds [2]. For O on H⁺₃, two flowing afterglow results at 300 K exist, but with rather large uncertainties and no information on the temperature dependence [3,4]. We have developed a novel merged beam apparatus to study reactions of neutral atomic C and O with molecular ions at the low collision energies relevant for molecular cloud studies. Photodetachment of atomic anion beams, with an 808-nm (1.53-eV) laser beam, is used to produce beams of neutral atomic C and O, each in their ground term as occurs in molecular clouds. The neutral beam is then merged with a velocity matched, co-propagating H⁺₃ beam. The merged beams method allows us to use fast beams (keV in the lab frame), which are easy to handle and monitor, while being able to achieve relative collision energies down to ≈ 10 meV. Using the measured merged beams rate coefficient, we are able to extract cross sections which we can then convolve with a Maxwellian energy spread to generate a thermal rate coefficient for molecular cloud temperatures. Here we report recent results for reactions of C and O on H⁺₃. Our reaction studies will help to provide a better basis for astrochemical models and benchmarks for future theoretical development. [1] Herbst and Millar, "Low Temperatures and Cold Molecules", ed. I. Smith (Imperial College Press, London), 2008, pp. 1-56. [2] Savić et al., Int. J. Mass. Spectrom., 2005, 240, 139-147. [3] Fehsenfeld, Astrophys. J., 1976, 209, 638-639. [4] Milligan & McEwan, Chem. Phys. Lett., 2000, 319, 482-485.

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