Laser photofragment study of isomeric C4H6+ cations at 5145 Å

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 Å 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.