Absolute unimolecular decay rates of energy selected C4H6+ metastable ions

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Abstract
The rate constants for the unimolecular decomposition of energy selected C4H6+ ions have been measured using the recently developed technique of zero kinetic energy (ZKE) photoelectron–photoion coincidence spectroscopy in conjunction with time-of-flight (TOF) mass spectrometry. The C4H6+ ions were prepared from 1,3-butadiene, 1,2-butadiene, cyclobutene, 2-butyne, and 1-butyne by photoionization. Absolute rate constants for unimolecular decay were determined from the TOF distributions of mass selected C3H3+ daughter ions. For parent metastables prepared with energies between 0.05 and 0.5 eV above the appearance potentials for the lowest fragmentation process, decomposition to C3H3+ and CH3, total unimolecular decay lifetimes were in the range 0.5–4 μsec. The experimental TOF decay curves were found to be exponential, with rate constants increasing monotonically with increasing excitation energy. These measurements also indicate that parent C4H6+ metastables rearrange to a common structure regardless of the neutral precursor. A comparison of the experimental rate constants with the predictions of the quasiequilibrium theory (QET) reveals a discrepancy of over two orders of magnitude near the dissociation threshold. The evidence suggests that the large experimentally determined threshold rate of 105 sec−1 is not associated with a kinetic shift, which was found to be negligible. In addition to the lifetime studies, ZKE photoelectron spectra, photoelectron–photoion coincidence spectra, and breakdown curves obtained directly from the coincidence measurements are presented for all five C4H6 isomers.