UC Berkeley

Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl and tert-butyl radicals. These radicals were produced in a collisionless environment from the flash pyrolysis of the appropriate precursor, nitrosobenzene for phenyl and azo-tert-butane for the tert-butyl radical. The photodissociation dynamics of the phenyl radical (C₆H₅) were investigated at 248 and 193 nm. At 248 nm, the only dissociation products observed were from hydrogen atom loss, attributed primarily to H + o-C₆H₄ (ortho-benzyne). The observed translational energy distribution was consistent with statistical decay on the ground state surface. At 193 nm, dissociation to H + C₆H₄ and C₄H₃ + C₂H₂ was observed. The C₆H₄ fragment can be either o-C₆H₄ or l-C₆H₄ resulting from opening of the phenyl ring. The C₄H₃ + C₂H₂ products dominate over the two H loss channels. Attempts to reproduce the observed branching ratio by assuming ground state dynamics were unsuccessful. This discord, between the experimentally observed branching ratio and the theoretically predicted branching ratio led us to reinvestigate the dissociation dynamics of the phenyl radical at 193 nm, while producing the radical under different source conditions.

The photodissociation dynamics of the tert-butyl radical (t-C₄H₉) were investigated at 248 nm. Two distinct channels of approximately equal importance were identified: dissociation to H + 2-methylpropene (C₄H₈), and CH₃ + dimethylcarbene (C₃H₆). Neither the translational energy distributions that describe these two channels nor the product branching ratio are consistent with statistical dissociation on the ground state, and instead favor a mechanism taking place on excited state surfaces. The studies presented in this dissertation show that although hydrogen atom loss is sometimes expected to be the only major dissociation pathway in the photodissociation of hydrocarbon radicals this is not always a justified assumption.