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Studies of the recombination of H and D atoms on graphite - "Laboratory Astrochemistry".
Molecules in interstellar gas play a fundamental role in astronomy. However, the formation of the simplest molecule, molecular hydrogen, is still not fully understood. At UCL (as part of the Centre for Cosmic Chemistry and Physics [CCCP]) we have developed an experiment to address these issues and explore the following questions: how is H2 formed on dust grain surfaces? What is the budget between internal, kinetic and surface energies in the formation process? What are the astronomical consequences of these results? H2 is the most abundant molecule in the Universe and is the key partner in almost all reactions of cosmic chemistry; therefore it is fair to say that the most important reaction involving cosmic dust grains is the formation of molecular hydrogen from incident atomic hydrogen. It has now become possible to investigate in the laboratory reactions on surfaces at low temperature and low pressure that mimic those occurring on cosmic dust. Our aim at the UCL-CCCP is to understand the nature of those reactions, to determine their efficiency in the interstellar medium, their energy budget, and their consequences in the interstellar medium. In 1997 we began a combined programme of experimental and theoretical work to study the formation of H2 and HD from atomic hydrogen and deuterium on astrophysically relevant surfaces. The experiments focus on measuring the internal energy content of newly-formed hydrogen. Our experimental technique involves continuously irradiating a graphite (HOPG) surface, held at 13K, with beam(s) of incident H (and D) atoms in a UHV environment. These atomic beams are generated by microwave dissociation of H2 (D2). The nascent product molecules desorbing from the surface are detected by state-selective laser ionization and the resulting signals can be transformed into the relative populations of the nascent ro-vibrational states formed at the surface. We have detected H2 molecules in their first (v=1) and second (v=2) excited vibrational states and HD molecules formed in v=1-4 when the target temperature is 15 K. Qualitative indications are that the relative number density of nascent HD increases with vibrational excitation over the states we have detected and that a considerable amount of the energy released on forming the H-H bond flows into the surface. We are about to implement an experimental extension to determine the translational energy of the nascent molecules as well as their ro-vibrational excitation. In collaboration with colleagues in the Physics Department, we are incorporating our results into chemical models of H2 formation in the interstellar medium and are planning an observational campaign to search for infrared emission from these vibrationally excited nascent molecules.
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