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.
Using a Hei line 58.4 nm source lamp, photoelectron angular distributions were measured for the series of four olefins: ethylene, isobutylene (2-methyl-propene), trimethylethylene (2-methyl-2-butene), and tetramethylethylene (2,3-dimethyl-2-butene). From these, the asymmetry parameter 13 as a function of photoelectron energy was obtained for each of these molecules. The following important effects in the behavior of 13 are observed: (a) In the 1r orbital ionization regions of the spectrum of each molecule, 13 increases with increasing electron energy across the vibrational envelopes. (b) With increasing methyl substitution (and at a fixed photoelectron enegy) 13 for this band decreases. (c) In the region of the spectra of each of the methyl-substituted ethylenes involving several 2pu bands, this energy dependence of 13 behaves as if they constituted a single band, in spite of the widely differing orbital symmetries. (d) Over most of the 2pu region of each molecule, 13 decreases with increasing photoelectron energy, except for the high ionization potential end of this region, where 13 increases instead. We attribute effects (b) and (d) to u-1r orbital mixing.
The kinetic energy release distributions in the fragmentation C4H6+→C3H3++CH3 at selected energies are reported and are compared with the analogous distributions from ’’metastable’’ C4H6+ and from C2H2++C2H4 collision complexes. The data seem to reflect the role of angular momentum in the collision experiments, as predicted by the Langevin model of the quasiequilibrium theory.
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