Unimolecular reaction dynamics from kinetic energy release distributions. III. A comparative study of the halogenobenzene cations

The translational kinetic energy release distribution (KERD) in the halogen loss reaction of the chloro-, bromo-, and iodobenzene cations has been exp erimentally determined in the microsecond time scale and theoretically anal yzed by the maximum entropy method. The KERD is constrained by the square r oot of the translational energy, i.e., by the momentum gap law. This can be understood in terms of quantum-mechanical resonances controlled by a matri x element involving a localized bound state and a rapidly oscillating conti nuum wave function, as in the case of a vibrational predissociation process . The energy partitioning between the reaction coordinate and the set of th e remaining coordinates is nearly statistical, but not quite: less translat ional energy is channeled into the reaction coordinate than the statistical estimate. The measured entropy deficiency leads to values of the order of 80% for the fraction of phase space sampled by the pair of fragments with r espect to the statistical value. In the case of the dissociation of the chl orobenzene ion, it is necessary to take into account a second process which corresponds to the formation of the chlorine atom in the excited electroni c state P-2(1/2) in addition to the ground state P-2(3/2). The observations are compatible with the presence of a small barrier (of the order of 0.12 eV) along the reaction path connecting the (D) over tilde (2)A(1) state of C6H5Cl+ to the Cl(P-2(1/2))+C6H5+ ((X) over tilde (1)A(1)) asymptote. (C) 1 999 American Institute of Physics. [S0021-9606(99)00906-X].