Photodissociation dynamics of the propargyl radical

The photochemistry and photodissociation dynamics of the propargyl radical, C3H3, upon UV excitation is investigated by time- and frequency-resolved d etection of hydrogen atoms. From a statistical analysis of the data, we con clude that formation of cyclopropenylidene, c-C3H2, is the dominant reactio n channel. Around 22% of the excess energy is released into the translation al 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 a s a function of excess energy. The experimental rates, on the order of seve ral 10(6) s(-1), are in good agreement with Rice-Ramsperger-Kassel-Marcus ( RRKM) calculations, provided a scaling factor is used for the vibrational f requencies, to account for the effects of anharmonicity. The interpretation is confirmed in experiments using monodeuterated propargyl radicals, H2CCC D, indicating a mechanism that proceeds via an initial [1,2] H-shift, follo wed by cyclization. (C) 1999 American Institute of Physics. [S0021-9606(99) 01232-5].