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Production and Photodissociation of Neutral Free Radicals


The primary photochemistry of several combustion-relevant free radicals have been in- vestigated via photofragment translational spectroscopy. The relevance of radical photo- chemistry will be discussed, along with methodologies and details of each experiment. The experimental apparatus will also be described, especially with regard to the recent installa- tion of a tunable energy electron ionizer. The upgraded ionizer has been a significant advance, allowing for more detailed characterization of the radical source employed in this thesis.

The photochemistry of the phenyl radical (c-C6H5), a combustion intermediate and pre- cursor to polycyclic aromatic hydrocarbons, was investigated at 248 and 193 nm. At 248 nm, an H-atom loss pathway was found, while at 193 nm both H-atom loss and C2H2 loss pathways were observed. For both wavelengths, P(ET) distributions suggested internal con- version to the ground electronic state followed by energy randomization and dissociation. The branching ratio between the two 193 nm dissociation pathways was found to be 0.2 ± 0.1 in favor of H-atom loss, in good agreement with statistical Rice–Rampsperger–Kassel–Marcus (RRKM) theory.

An initial investigation of the methyl perthiyl radical (CH3SS) at 248 nm suggested the surprising results of both CH3 + SS and CH2S + SH dissociation channels with no evidence for S-atom loss. In both cases, the translational energy distributions were inconsistent with the expected energetics. Upon reinvestigation, the assumption of radical production—and there- fore radical photodissociation—was shown to be incorrect. The new results demonstrated S-loss and CH3 loss pathways, with the former appearing to involve a repulsive electronic excited state.

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