UC Berkeley

The photodissociation of several combustion relevant radical molecules has been carried out. These species behave significantly different from closed shell species and the experiments presented here provide fundamental new insights into the chemical dynamics of free radicals.

The experiments were carried out on a molecular beam photofragment translational spectroscopy instrument. Radicals were generated by flash pyrolysis of a suitable precursor and entrained in a pulsed molecular beam. The radicals were then photodissociated and the products were detected with a rotatable mass spectrometer based on electron impact ionization.

The benzyl radical photodissociation and the cyclopentadienyl radical photodissociation were investigated at 248 nm. Two dissociation channels were observed for each radical. The benzyl radical dissociates into H + C₇H₆ and CH₃ + C₆H₄ and the cyclopentadienyl radical dissociates to H + C₅H₄ and C₃H₃ + C₂H₂, where the C₅H₄ fragment was identified as ethynylallene by its ionization energy.

The translational energy distribution determined for each channel suggests that every dissociation mechanism occurs via internal conversion to the ground state followed by intramolecular vibrational redistribution and dissociation. The branching ratio between the two competing channels for each radical was measured experimentally and compared to RRKM (Rice-Ramspberger-Kassel-Marcus) calculations. For both radicals, the dominant experimental channel was corroborated by RRKM calculations.