Laser photofragment study of isomeric C4H6+ cations at 5145 A

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 A beam of an argon ion laser and the C3H3+ photofragments, the only fragments observable, are energy-analyzed by an electrostatic analyzer. On the basis of C3H3+ photofragment recoil spectra, we argue that five isomerically distinct neutrals 1,3-butadiene, 1,2-butadiene, cyclobutane, 1-methylcyclopropene, and 3-methylcyclopropene, form parent ions which appear to rearrange to a common precursor prior to fragmentation. Neither 1-butyne nor 2-butyne cations photodissociate at this wavelength. The dienes 1,3-butadiene and 1,2-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 100--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 1,3-butadiene ions at comparable excess energies, in which substantially lower kinetic energy releases, consistent with a statistical theory, were observed [Klots, Mintz, and Baer, J. Chem. Phys. 66, 5100 (1977)]. 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