Low Temperature Kinetic Studies using a Pulsed Laval Nozzle Apparatus

Laboratory kinetic studies of reactions relevant to interstellar environments have been performed using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis-laser induced fluorescence spectroscopy in the temperature range 54-148 K. Rate coefficients for the reactions of the hydroxyl radical with several oxygenated organic molecules are reported in Chapters 3 and 4. At low temperatures, the rate coefficients for these reactions are found to be significantly enhanced despite barriers to hydrogen abstraction. A common mechanism has been identified involving the initial formation of a weakly bound complex (~ 15-30 kJ mol-1), which has an extended lifetime at lower temperatures. The extended lifetime of the complex facilitates two competing channels: collisional stabilisation into the pre-barrier well, or quantum mechanical tunnelling through the hydrogen abstraction barrier. The role of these channels is assessed through studies of pressure dependence. The mechanism is also found to be operative for even very weakly bound complexes, such as in the reaction of OH with ammonia as reported in Chapter 5. Pressure dependence and product detection studies enable the low temperature yield of NH2 radicals from this reaction to be quantified. The potential interstellar implications of these reactions in light of the rate coefficients obtained in this work are reported. In Chapter 6, the reaction of the postulated products from the OH + methanol and ethanol reactions, methoxy and ethoxy radicals, with NO are studied for the first time at low temperatures. The role of pressure stabilization of the complex, RONO, versus bimolecular product formation is investigated through pressure dependence studies and detection of the NO + methoxy radical product, HCHO, via laser induced fluorescence spectroscopy. The low temperature high pressure limiting rate coefficients of the OH + oVOC and OH + NH3 reactions are explored in Chapter 7 using the proxy method of Jaffer, Smith, Quack and Troe. Rate coefficients for the reactions are obtained at different quanta of vibrational excitation of OH, and the validity of the proxy method for weakly bound complexes at low temperatures is considered with regards to efficient intramolecular vibrational relaxation.