Kinetic energy release distributions for the dissociation of internal energy selected CH₃I⁺ and CD₃I⁺ ions

Product translational energy distributions have been measured from an analysis of symmetrically broadened time‐of‐flight peaks of CH₃⁺ and CD₃⁺ ions in photoion–photoelectron coincidence spectra of CH₃I and CD₃I, respectively. The reactions were investigated at various precursor ion internal energies up to 1.7 eV above the dissociation threshold. We found that the experimentally obtained kinetic energy release distributions can be fitted accurately by simple exponential functions over the energy range considered. The experimental results are compared to predictions of the statistical theory of unimolecular reactions. An angular momentum conserving version of the quasiequilibrium theory, formulated in terms of the Langevin collision cross section model for the reverse association reaction, predicts kinetic energy release distributions which agree very well with the experimental results at precursor ion energies up to 0.65 eV above the dissociation onset. At approximately this energy, the average kinetic energy released begins to deviate sharply from the theoretically predicted average, possibly indicating the accessibility of a new reaction channel.