Photoionization of Isomers. Unimolecular and Bimolecular Reactions of the Ions

1-Butene, cis-2-butene, isobutene and methylcyclopropane have been photoionized with the resonance lines of krypton (10.0-10.6 eV) and argon (11.6-11.8 eV). We have determined that the internally excited $$ ion and, to a much lesser extent, the $$ ion isomerizes to the $$ structure. In both cases the extent of isomerization increases, approximately by a factor of ten when the photon energy is increased from 10 to 11.7 eV. An inert gas, neon, quenches the isomerization of the $$ ion and, to a much lesser rextent, that of the $$ ion. The unimolecular fragmentation of the $$ isomeric ions has been examined at 11.61-11.8 eV. In this energy range the dissociative lifetime of $$ was found to be at least 5 × 10^-6 s, and collisional quenching of the dissociative process is already noticeable at pressures in the 10^-3 torr range. The rate coefficients for the reaction $$ occurring in the isomeric C₄H₈ systems have been determined under conditions where the structure of the reacting $$ ion is established. The values in cm³/molecule · second are 1-C₄H₈ - 6.0 ± 0.5 × 10^-l0, cis-2-C₄H₈ - 0.37 ± 0.1 × 10^-10, i-C₄H₈ - 5.4 ± 0.4 × 10^-10. At pressure below 10^-3 torr, the internally excited $$ produced in the reaction dissociates along various channels with relative probabilities depending upon the structure of both the ionic and neutral reactant. Above 10^-3 torr collisional quenching of $$ is noted.