Photoionization of Propylene, Cyclopropane, and Ethylene. The Effect of Internal Energy on the Bimolecular Reactions...

The ion-molecule chemistry occurring in photoionized ethylene, propylene and cyclopropane has been investigated in the NBS high pressure mass spectrometer. Emphasis has been placed on determining the overall effect of internal excitation on the absolute rate coefficients for the various reactions. This was accomplished by comparing the behavior found when ionization was induced by photon absorption at the ionization threshold with that found at higher photon energies. The values for the absolute rate coefficients for the formation of excited compound ions at various photon energies were as follows (units of 10^-10 cm³/molecule · s); $$ , 9.6 (10.6 eV photons); 8.5 (11.7 eV photons);. $$ , 8.4 (10.0 eV photons); 6.8 (11.7 eV photons); $$ , 7.8 (10.0 eV photons), 6.1 (11.7 eV photons); $$ and $$ , 2.2 (10.0 eV photons); 2.2 (11.7 eV photons). The distributions of product ions obtained from the unimolecular decomposition of the compound ion in each system have also been determined at various photon energies, and considerable variation has been observed. In general, the variations are similar to those found elsewhere when the kinetic energy of the reactant ion is increased. The bimolecular reactions of the $$ compound ion obtained from $$ > have been investigated at higher pressures, and separate experiments involving isotopically labeled ethylenes have provided new information concerning the nature of the compound ion in that system. Results obtained from other laboratories are discussed in some detail, and in most cases the differences found for the same system are explicable in terms of the energy content (internal and/or kinetic) of the reactant ion.