Kinetic energies of ions produced by dissociative electron impact ionization of propane

Mass spectrometric techniques in conjunction with the ion deflection method have been used to measure kinetic energy spectra of the various ions produced by electron-impact ionization and dissociative ionization of propane, C₃H₈. The kinetic energy spectra of the C₃H⁺_i ions (i = 0-8) indicate that these ions are almost exclusively formed with quasithermal energies and that the mechanism of their formation is dominated by the removal of one or more neutral H and H₂ fragments accompanied in some cases by a molecular rearrangement of the residual fragment ion. The spectra of the C₂H⁺_i fragment ions (i = 0-5) show, besides a quasithermal peak, also ions with higher kinetic energies indicating that a fraction of the ions are the result of processes favoring the formation of energetic, nonthermal fragment ions. Although the quasithermal contribution is dominant in the spectra of the C₂H⁺₅ and C₂H⁺₄ fragment ions, the spectra of the smaller C₂H⁺_i ions (i = 0-3) show a dominance of energetic, nonthermal ions whose formation most likely proceeds via the initial excitation of high-lying repulsive target states. The spectra of the CH⁺_i fragment ions (i = 1-3) are dominated by the presence of energetic, nonthermal ions with kinetic energies of up to 4 eV per fragment ion with quasithermal ions accounting for only about 15% or less of the total ion signal.