Photodissociation of the Cyclopentadienyl Radical at 248 nm

The photodissociation of jet-cooled cyclopentadienyl radicals, c-C₅H₅, at 248 nm was studied using photofragment translational spectroscopy. Two dissociation channels were observed: H + C₅H₄ and C₃H₃ + C₂H₂. The C₅H₄ fragment was identified as ethynylallene by its ionization energy. The translational energy distribution determined for each channel suggests that both dissociation mechanisms occur via internal conversion to the ground electronic state followed by intramolecular vibrational redistribution and dissociation. The experimental branching ratio and RRKM (Rice-Ramsperger-Kassel-Marcus) calculations favor the formation of C₃H₃ + C₂H₂ over the H-atom loss channel. The RRKM calculations also support the observation of ethynylallene as the dominant C₅H₄ product isomer.