Photoisomerization and photodissociation dynamics of reactive free radicals

The photofragmentation pathways of chemically reactive free radicals have been examined using the technique of fast beam photofragment translational spectroscopy. Measurements of the photodissociation cross-sections, product branching ratios, product state energy distributions, and angular distributions provide insight into the excited state potential energy surfaces and nonadiabatic processes involved in the dissociation mechanisms. Photodissociation spectroscopy and dynamics of the predissociative {tilde A}{sup 2}A{sub 1} and {tilde B}{sup 2}A{sub 2} states of CH{sub 3}S have been investigated. At all photon energies, CH{sub 3} + S({sup 3}P{sub j}), was the main reaction channel. The translational energy distributions reveal resolved structure corresponding to vibrational excitation of the CH{sub 3} umbrella mode and the S({sup 3}P{sub j}) fine-structure distribution from which the nature of the coupled repulsive surfaces is inferred. Dissociation rates are deduced from the photofragment angular distributions, which depend intimately on the degree of vibrational excitation in the C-S stretch. Nitrogen combustion radicals, NCN, CNN and HNCN have also been studied. For all three radicals, the elimination of molecular nitrogen is the primary reaction channel. Excitation to linear excited triplet and singlet electronic states of the NCN radical generates resolved vibrational structure of the N{sub 2} photofragment. The relatively low fragment rotational excitation suggests dissociation via a symmetric C{sub 2V} transition state. Resolved vibrational structure of the N{sub 2} photofragment is also observed in the photodissociation of the HNCN radical. The fragment vibrational and rotational distributions broaden with increased excitation energy. Simple dissociation models suggest that the HNCN radical isomerizes to a cyclic intermediate (c-HCNN) which then dissociates via a tight cyclic transition state. In contrast to the radicals mentioned above, resolved vibrational structure was not observed for the ICNN radical due to extensive fragment rotational excitation, suggesting that intermediate bent states are strongly coupled along the dissociation pathway. The measurements performed in this Thesis have additionally refined the heats of formation and bond dissociation energies of these radicals and have unambiguously confirmed and added to the known electronic spectroscopy