Kinetics of astrochemical reactions at very low temperatures

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The chemical mechanisms of key gas-phase astrochemical reactions using both experimental and computational methods will be explored. The experimental part of the project involves using a pulsed Laval nozzle apparatus using laser flash-photolysis combined with laser-induced fluorescence (LIF) spectroscopy to study the kinetics of reactions down to temperatures around 25 K (the current limit of our apparatus), close to those encountered in interstellar space. Rate coefficients, k(T), will be measured for reactions of small free-radicals, for example OH, carbon-based radicals (e.g. CN) and nitrogen-based radicals (e.g. NH2) with organic molecules found in space containing a range of functional groups. To complement the experiments, the potential energy surfaces (PESs) for these reactions will be calculated by ab initio methods utilising the Gaussian suite of computer programmes, or other methods. The PESs will then be used to compute rate coefficients as a function of temperature and pressure, using the MESMER rate theory software package developed at Leeds. MESMER will also allow rate coefficients to be calculated outside of achievable experimental conditions, for example down to 10 K, and will enable the calculation of product branching ratios as a function of temperature and pressure. A comparison of theoretical predictions with experimental measurements will enable details of the PESs to be optimised and hypotheses for reaction mechanisms to be validated. The project will also develop parameterisations for k(T) and use these in astrochemical models to calculate the abundance of complex organic molecules for comparison with astronomical telescope observations. This will improve our understanding of the chemical processes in space which control the formation of destruction of complex organic molecules, some of which may link to prebiotic molecules, which account for their abundance.