<p> When excited in the region 365-436 nm in the gas phase, 2,3-pentanedione is shown to emit fluorescence and phosphorescence. The absolute emission yields have been determined and the radiationless processes removing the excited singlet and triplet states have been considered. </p> <p> The radiationless processes removing the triplet state have been investigated in different temperatures and concentrations and are shown to be intersystem crossing to the ground state, a temperature dependent unimolecular reaction and a temperature dependent bimolecular self-quenching reaction. </p> <p> Excitation at 365 nm causes emission from vibrationally excited levels of the singlet state. The variation of the fluorescence yield with pressure at this excitation wavelength is compatible with either a strong or weak collision mechanism for vibrational deactivation within the singlet manifold. Phosphorescence yield measurements support this conclusion. </p> <p> A consistent mechanism is proposed to describe the system and is tested with fluorescence, phosphorescence, and lifetime measurements at various temperatures and concentrations. </p> <p> The interaction of the triplet state of pentanedione with various classes of substrate has been investigated. Emphasis has been placed on temperature dependence of the reaction modes. The possibility of energy transfer from the triplet pentanedione molecule has been investigated with 1,3-butadiene and cyclopentadiene at different temperatures. The butadiene quenching system is kinetically simple at all temperatures studied, whereas the cyclopentadiene is simple at high temperatures but complex at lower temperatures. This behaviour has been reconciled with reversible energy transfer in the gas phase triplet pentanedione/cyclopentadiene system at the lower temperature. A mechanism is proposed which demonstrates directly the intermediacy of the triplet cyclopentadiene species and, therefore, confirms the energy transfer nature of the interaction. The energy transfer rate constants are discussed with respect to the current models of energy transfer to dienes. </p> / Thesis / Doctor of Philosophy (PhD)