Laser photofragment kinetics
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
We report here several studies of the kinetics of reactions of free radical species in the gas phase. The radicals were produced by ultraviolet laser photolysis of stable precursor molecules in the presence of various reactant gases together with an excess of inert bath gas. The ensuing reactions were followed by means of either time-resolved laser-induced fluorescence (LIF) measurements of relative radical concentrations, or time-resolved resonance fluorescence measurements of the relative concentrations of product atoms. From the LIF measurements, bimolecular rate coefficients have been obtained at room temperature for the reactions CN + N → N₂ + C [(1.00 ±0.13) x 10⁻¹⁰ cm³s⁻¹], CN + O₂ → products [(1.27 ±0.23) x 10⁻¹⁰ cm³s⁻¹], NH₂ + N → products [(1.21 ±0.14) x 10⁻¹ⁱ cm³s⁻¹], NH₂ + NO → products [(1.81 ±0.12) x 10⁻¹ⁱ cm³s⁻¹], NH₂ + NO₂ → products [(2.11 ±0.18) x 10⁻¹ⁱ cm³s⁻¹], NH + NO → products [(5.78 ±0.64) x 10⁻¹ⁱ cm³s⁻¹], NH + NO₂ →products [(1.61 ±0.14) x 10⁻¹ⁱ cm³s⁻¹]. The rate coefficients for the reactions of NH have also been determined over the temperature range 269 - 377 K, yielding activation energies of 0 ±2 kJ mol⁻¹ (NH + NO) and - 9.5 ±3.2 kJ mol⁻¹ (NH + NO₂). In addition to the rate coefficient measurements the nature of the products of the NH₂ + N reaction has been investigated by measuring the relative yields of product hydrogen atoms via resonance fluorescence. The time dependence of the H atom concentration is consistent with production of H by the reaction NH₂ + N → N₂ + H + H. The same method has been used to establish that H atoms are not produced in the reaction between NH₂ + NO. These results have been interpreted in relation to the mechanisms of the reactions concerned. In particular, the nature of the primary products of the NH₂ + NO reaction is still uncertain; the present results on the NH₂ + NO and NH + NO systems indicate that OH is not a primary product of the former reaction, contrary to previous suggestions. This uncertainty and the results of a recent theoretical study of the kinetics of the NH₂ + NO reaction have raised questions about the nature of the reaction potential energy surface. Using the methods of ab initio molecular orbital theory we have determined the structures, relative energies and vibrational frequencies of a range of species which occur along the reaction coordinates leading to several alternative sets of products. The results of this investigation have been compared to those of earlier experimental and theoretical studies of the NH₂ + NO reaction.