Thesis (M. Phil.)--University of Hong Kong, 1995. / Includes bibliographical references (leaf 90-94).

The measurement of radical species in the atmosphere has far reaching implications. For example, it is necessary to both understand and improve our knowledge of radicals in the atmosphere to better inform the models which in many cases are the best way of predicting future air quality and climate change. Although many of these models are often not fully representative of all the processes occurring, they are the current best estimate based on the knowledge available, and can be useful in informing and directing future policy. The numerous, varied and interlinked cycles in the atmosphere are complex and only by obtaining data on specific species can accurate concentrations be retrieved and fed back into the models to improve their accuracy. This work is concerned with the development and application of an ultrasensitive absorption spectroscopy technique to the problem of detection of the peroxy radical, HO₂. Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) combines cavity enhancement techniques (in order to increase the path length) with frequency and wavelength modulation techniques (in order to reduce the noise). Following a discussion of the current detection methods used by atmospheric scientists to accurately measure and quantitative concentrations, some preliminary work on the detection of ammonia by a simple cavity enhanced absorption setup is presented. Pressure broadening and shift results were obtained for a number of ammonia transitions in the near infrared region, broadened by He, Ne, Ar, Xe, O₂ and N₂. The bulk of the work concentrates on the implementation of the NICE-OHMS technique, presenting the first results with the use of an external cavity diode laser and a ring shaped cavity. A sensitivity of 4 x 10⁻¹¹ cm⁻¹ Hz⁻^1/2 is obtained on an individual rovibrational transition of methane at 6610.063 cm⁻¹, along with a selection of other data from the atmospherically important molecules methane, nitrogen dioxide and carbon dioxide, highlighting the broad wavelength range over which the instrument can operate. Finally, the NICE-OHMS technique is used to probe HO₂ radicals formed through the photolysis of a Cl₂/CH₃OH/O₂ mixture. Following the creation and detection of HO₂ radicals in the cavity, and based on the optimum sensitivity outlined above, a minimum concentration of 1 x 10⁹ molecules cm⁻³ has been demonstrated.

This thesis describes the combination of two sources of cold atomic or molecular species which can be used to study a wide range of ion-molecule reactions. The challenges in forming these species and in determining the fate of reactive events are explored throughout. Reactions occur in a volume within a radio-frequency ion trap, in which ions have previously been cooled to sub-Kelvin temperatures. Ions are laser-cooled, with migration of ions slowed sufficiently to form a quasi-crystalline spheroidal structure, deemed a Coulomb crystal. Fluorescence emitted as a consequence of laser-cooling is detected; the subsequent fluorescence profiles are used to determine the number of ions in the crystal and, in combination with complementary simulations, the temperature of these ions. Motion imparted by trapping fields can be substantial and simulations are required to accurately determine collision energies. A beam of decelerated molecules is aimed at this stationary ion target. An ammonia seeded molecular beam enters a Stark decelerator, based on the original design of Meijer and co-workers. The decelerator uses time-varying electric fields to remove kinetic energy from the molecules, which exit at speeds down to 35 m/s. A fast-opening shutter and focussing elements are subsequently used to maximise the decelerated flux in the reaction volume while minimising undecelerated molecule transmission. Substantial fluxes of decelerated ammonia are obtained with narrow velocity distributions to provide a suitable source of reactant molecules. Combination of these two techniques permits studies of reactions between atomic ions and decelerated molecules that can be entirely state-specific. Changes in the Coulomb crystal fluorescence profile denote changes in the ion identities, the rate of these changes can be used to obtain rate constants. Determination of rate constants is even possible despite the fact that neither reactant nor product ions are directly observed. This work has studied reactions between sympathetically cooled Xe⁺ ions and guided ND3 and has obtained data consistent with prior studies. Determination of reactive events is complicated if ion identities can change without affecting the fluorescence profile, or if multiple reaction channels are possible. A range of spectroscopic techniques are discussed and considered in regards to determining rate constants and product identities. Pulsed axial excitation of trapped ions can follow rapid changes in average ion weights and subtle changes for small crystals. Time-of-flight mass spectrometry is also demonstrated using the trapping electrodes and is suitable for discrimination of ions formed within the trap.