The role of ortho-hydrogen in reactions of interstellar relevance
Dieter Gerlich
Faculty of Natural Science, Technical University, 09107 Chemnitz,
Germany
Abstract
The chemistry of the interstellar medium is quite different from other
chemical
systems since low densities and low temperatures prevail. Therefore,
small
endothermicities or barriers as well as excitation of the reactants to
low lying
fine structure or rotational states can change significantly the outcome
of a
reactive collision. For example the astrochemically important process of
isotope enrichment (e.g. deuteration) is caused by a gain in zero point
energies; however, recent experiments prove that such reactions often do
not
reach thermodynamical equilibrium, especially if traces of
ortho-hydrogen are
present.
This talk will first review a few basic beam experiments, where o-H2
plays a
role and summarize a some theoretical arguments (symmetry selection
rules) why
the nuclear spin can influence chemical reactions. In recent years many
experiments "under inter-stellar conditions" have been performed using
temperature variable ion traps. This method which uses a radio frequency
22-pole
field for confining the ions will briefly explained together with a few
typical
applications. Detailed results will be presented for the deuteration of
Hn+ and
several hydrocarbons CmHn+. The outlook will give some hints how to
extend the
trapping method into different directions (astrochemistry, sub-K range,
nested
trap for ion-electron recombination).