High-resolution analysis of the kinetic energy distribution of fragment ions produced by dissociative ionization of propane

We report measurements of the kinetic energy distributions of fragment ions produced by electron impact dissociative ionization of propane using a two-sector-field mass spectrometer in conjunction with the retarding field method. This technique achieves a higher energy resolution, albeit at much reduced ion signals and at a significant loss of energetic fragment ions compared to the conventional ion deflection technique, which has been used extensively by other groups in the past. The higher energy resolution used in the present experiments results in a complete separation of the thermal/quasi-thermal fragment ions from the energetic fragment ions. This, in turn, reveals that the energetic fragment ions have a much narrower kinetic energy distribution than previously assumed. Because of the comparatively weak ion signals, further studies such as separate threshold cross-section measurements and appearance energy determinations for, respectively, the "slow" and "fast" fragment ions could only be carried out for the CH₃⁺ and C₂H₄⁺ fragment ions.