The photochemistry and photodissociation dynamics of the propargyl radical, C_3H_3, upon UV excitation is investigated by time- and frequency-resolved detection of hydrogen atoms. From a statistical analysis of the data, we conclude that formation of cyclopropenylidene, c-C_3H_2, is the dominant reaction channel. Around 22% of the excess energy is released into the translational degrees of freedom. By varying the excitation energy between 265 and 240 nm, microcanonical rates for the loss of a hydrogen atom can be obtained as a function of excess energy. The experimental rates, on the order of several 10~6 s~(-1), are in good agreement with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations, provided a scaling factor is used for the vibrational frequencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H_2CCCD, indicating a mechanism that proceeds via an initial [1,2] H-shift, followed by cyclization.
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