We present a study of the isomerization of seven isomeric C4H6+cations using laser photofragment spectroscopy. Parent ions created by electron impact intersect the 5145 Aring; beam of an argon ion laser and the C3H3+photofragments, the only fragments observable, are energyhyphen;analyzed by an electrostatic analyzer. On the basis of C3H3+photofragment recoil spectra, we argue that five isomerically distinct neutrals 1,3hyphen;butadiene, 1,2hyphen;butadiene, cyclobutane, 1hyphen;methylcyclopropene, and 3hyphen;methylcyclopropene, form parent ions which appear to rearrange to a common precursor prior to fragmentation. Neither 1hyphen;butyne nor 2hyphen;butyne cations photodissociate at this wavelength. The dienes 1,3hyphen;butadiene and 1,2hyphen;butadiene show a minor fragmentation channel to a higher energy product, which we hypothesize to be the propargyl cation. The very high kinetic energy release upon fragmentation (most probable values of 100ndash;200 MeV) cannot be rationalized on the basis of a statistical model of vibrational predissociation in the ground electronic state. The results are also inconsistent with PIPECO fragmentation studies of 1,3hyphen;butadiene ions at comparable excess energies, in which substantially lower kinetic energy releases, consistent with a statistical theory, were observed lsqb;Klots, Mintz, and Baer, J. Chem. Phys. 66, 5100 (1977)rsqb;. We argue that the high translational energy release observed here is consistent with excitation to the first excited state of the parent cation on a portion of the potential surface, which is coupled strongly to a repulsive potential whose barrier is preferentially disposed of in product kinetic energy.
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