Chemical Physics and Astrochemistry of H + 3 and Sub Doppler Mid

Molecular ions are known to be key reactive intermediates in interstellar environments, and H3 in particular is responsible for initiating a network of chemical reactions that ultimately results in the formation of the ∼170 molecules detected so far in space. Yet fundamental questions about the interstellar abundances of the two nuclear spin configurations of H3 (ortho-H + 3 , I = 3/2; and para-H + 3 , I = 1/2) remain. In this thesis, experiments to measure the nuclear spin dependence of the chemical reactions of H3 with electrons and with molecular hydrogen at astronomically relevant temperatures are described. The results of these laboratory measurements are included into a model of the hydrogenic chemistry of diffuse molecular clouds, in which an excess of para-H3 is observed relative to its expected abundance at the measured cloud temperature. The model suggests that the ortho:para ratio is likely controlled by a competition between the aforementioned chemical reactions of H3 with electrons and molecular hydrogen. Other molecular ions may similarly be useful for constraining the physical and chemical conditions of astronomical environments, but such insight can only be derived if laboratory spectroscopy of these ions has been performed. However, for many ionic species, insufficient laboratory data are available, and this is primarily because of difficulties in producing sufficient quantities of ions for traditional spectroscopic techniques. This thesis discusses the development of instrumentation to overcome the challenge of ion production. First, a continuous supersonic expansion discharge source is described that allows for the production of internally cold molecular ions in modest abundance, thereby maximizing population in the lowest-lying energy states relevant for astronomical spectroscopy. Then, an instrument for performing sub-Doppler spectroscopy of molecular ions in a liquid nitrogen cooled plasma is discussed. This instrument offers ultra high sensitivity, and sufficient accuracy and precision that rotational frequencies suitable for observational astronomy can be inferred. As a case study of the performance and capabilities of this instrument, the high resolution sub-Doppler spectrum of H3 is presented.