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.