Laboratory studies of the formation of molecular hydrogen on surfaces at cryogenic temperatures
The interstellar medium (ISM) is the region of space between the stars, where star and planet formation occurs. Molecular hydrogen in the ISM initiates all the chemistry that occurs in these regions and without it, no stars or planets could form. The molecules formed by the chemistry initiated by H2 provide cooling mechanisms for the huge molecular clouds which collapse to form stars. It is well known that the abundance of molecular hydrogen in the ISM is too high for it to form through gas phase processes alone and hence, the widely accepted theory is that H2 forms through heterogeneous catalysis on the surfaces of interstellar dust grains. These grains make up approximately 1% of the mass of the ISM and are thought to be carbonaceous or silicate in nature. Despite its importance, only recently have laboratory experiments been set-up to study the formation of molecular hydrogen on interstellar grain analogues in detail. This thesis presents results from an experiment designed to determine what happens to the energy released on the formation of H2, under conditions similar to those of the ISM. The experiment involves using an atom source, ultrahigh vacuum chamber and cryogenic cooling methods in order to reach the temperatures and pressures of interstellar space. The laser technique of Resonance Enhanced Multiphoton Ionisation (REMPI) is employed to look at the internal energy distribution of newly formed H2 and HD molecules from a graphite surface. Improvements to an existing experiment are described, including the introduction of a second atom source to study HD formation. New results are presented, including the first observations of molecular hydrogen formed ro-vibrationally excited in states v" = 1 and v" = 2. The results are placed in context with the results of other experiments to form H2 under ISM conditions, and the astrophysical implications are discussed.