Microcanonical rates for the unimolecular dissociation of the ethyl radical

We report results on the photodissociation dynamics of the ethyl radical, C2H5. A beam of internally cold radicals is generated by supersonic jet flash pyrolysis of ethyl iodide or n-propylnitrite, respectively. Upon excitation into the A (2)A(1) 3s Rydberg state ethyl dissociates, presumably on the ground-state surface, into ethylene and a hydrogen atom that is detected by time-resolved photoionization. By varying the excitation energy between 264 and 245 nm, microcanonical rates are obtained as a function of excitation energy. The reaction rates are on the order of 10(7) s(-1), several orders of magnitude slower than expected from simple Rice-Ramsperger-Kassel-Marcus calculations. (C) 1999 American Institute of Physics. [S0021-9606(99)02812-3].