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UBC Theses and Dissertations

Primary photochemical processes in hexafluorobiacetyl Reid, William John


Absolute photochemical quantum yields of hexafluoro-biacetyl vapour have been obtained at various exciting wavelengths between 250 and 440 nm over the range 0.5 - 400 torr. The yields are strongly dependent on pressure demonstrating that vibrational relaxation is the dominant process competing with unimolecular dissociation. It is found that two different states contribute to dissociation. One is identified as the excited singlet level reached on excitation. The other is attributed to the vibronic level reached on intersystem crossing from that initially formed vibronic state. The intersystem crossing rate constant has been shown to be a strong function of excitation energy. It is postulated that the first excited singlet state of hexafluorobiacetyl is photochemically inert unless it has at least 70 kcal of vibronic energy. This accounts for no decomposition being observed at the higher wavelengths. Temperature and quenching studies have shown that the equilibrated triplet state is unreactive photochemically. Phosphorescence lifetime measurements at very low pressures have confirmed that wall-deactivation for the relatively long lived equilibrated triplet species is important when the average distance which the triplet molecule can diffuse is of the same magnitude as the cell radius. The data from the various investigations are combined to give a description of the primary photochemical and photo-physical events. From this information a mechanism for the primary process in hexafluorobiacetyl is proposed and critically evaluated. Estimates of the specific rate constants for the photochemical processes are given and discussed.

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