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.
