Some Remarks on the Photophysics of Nitrogen Dioxide

A high kinetic energy resolution velocity map imaging spectrometer has been designed and constructed to perform energy- and time-resolved photoion or photoelectron imaging experiments. When coupled with fast detector gating the instrument allowed direct current slice ion imaging experiments to be performed. Alternatively crush imaging experiments could be performed with better mass resolution and greater signal intensities. The main body of the thesis focuses on the application of this spectrometer to the study of the ultraviolet fragmentation of nitrogen dioxide. A combination of direct current slice imaging, crush velocity map imaging and photofragment resonantly enhanced multiphoton ionisation spectroscopy experiments have been performed to understand the fragmentation dynamics. At an excitation energy of ~5.49 eV 0 (3Pj) fragments were formed preferentially in coincidence with NO (l)2n_q fragments in v=l with a bimodal rotational distribution. 0 ('D 2) fragments were observed to be formed preferentially with vibrationally excited NO ( l) 2rin (v=l) with a bimodal rotational distribution. In addition the polarisation
of the 0 ('D 2) fragments was extracted and the orbital angular momentum alignment of these fragments was found to be essentially independent of kinetic energy release. The results were interpreted in terms of the topology of the electronically
excited states of nitrogen dioxide. In a separate set of experiments the visible/ultraviolet decomposition dynamics of nitrogen dioxide were studied. Internally excited NO (l)2IIn fragments were observed to be formed and were probed using state-selective multiphoton ionisation schemes. Direct measurements of atomic oxygen fragments suggest that these fragments are produced via one or more high-lying neutral electronic states. In the case of ground state oxygen atoms, fragmentation results in the preferential production of highly vibrationally excited neutral NO co-fragments. Oi'So) fragments were also observed and interpreted to be produced in coincidence with NO (l)2IIn fragments. The relevance of these results to the interpretation of recent
time-resolved experiments was discussed.