Experimental studies of surface and gas-phase processes relevant to the interstellar medium and planetary atmospheres.

The interstellar medium (ISM) is the region of space between stars, where star and planet formation occurs. Molecular hydrogen is vitally important to the chemistry that happens in the ISM, as it initiates most of the reactions. The molecules that result from these reactions act as a coolant for the huge molecular clouds that collapse to form stars. The abundance of molecular hydrogen in the ISM is too high to form through gas phase processes alone. The accepted formation mechanism for H_2 is via heterogeneous catalysis on the surface of interstellar dust grains. These dust grains make up approximately 1% mass of the ISM and are carbonaceous or silicate in composition. The experiment presented in this thesis has been designed to probe ro-vibrational energy of nascent HD formed on an interstellar dust grain analogue. The experiment involves the use of two atom sources, to produce H and D atoms, ultrahigh vacuum chambers and cryogenic cooling to obtain conditions relevant to the ISM. The laser technique of resonance enhanced multiphoton ionisation (REMPI) is employed to state selectively ionise the HD, in order to investigate the internal energy of the newly formed HD when itdesorbs from the highly orientated pyrolitic graphite surface. The results are presented here and are compared to other theoretical and experimental studies; the astrophysical implications are discussed. Also included in this thesis is a study of the electron impact ionisation of OCS using pulsed time-of-flight in conjunction with a 2-D coincidence technique. The experimentalset-up allows fragment ions to be formed by dissociative single, double and triple ionisation to be detected and quantified. The relative partial ionisation cross sections (PICS) and precursor specific relative PICS are derived as well as information on OCS^{2+}. OCS is found in the atmospheres of Jupiter and Venus, where it may exist in a doubly charged state, OCS has been observed in the interstellar medium.