Kinetic Energies of Products of Ion—Molecule Reactions

The mean kinetic energies of the ionic products of the reactionsC2H4++C2H4→C3H5++CH3,C2H2++C2H4→C3H3++CH3,CH3++CH4→C2H5++H2have been measured in a mass spectrometer equipped with a cylindrical electrostatic energy analyzer. The translational energies observed in the center-of-mass frame of reference for Reactions (I), (II), and (III) were 0.085±0.05, 0.150±0.05, and 0.056±0.05 eV, respectively. The values are somewhat higher than the mean kinetic energies calculated from statistical theory by two different methods (one using the ``temperature'' approximation of Chupka, the other the semiclassical approximation of Klots), but for Reactions (I) and (III) the difference was less than the experimental error. For the collision complexes formed from these primary ions and molecules, the comparisons suggest that the fractions of the excitation energies that are partitioned among the translational modes of the dissociation products of the complexes are somewhat higher than the fractions estimated from a statistical theory that assumes complete equilibration of the excess energy among the many internal degrees of freedom of the complexes. Mathematical procedures for examining the kinematics of the ion—molecule collisions and subsequent dissociations of the complexes are described. Included are methods for eliminating the effects of instrumental bias on the experimentally determined translational energies.