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 met hod. This technique achieves a higher energy resolution, albeit at much red uced ion signals and at a significant loss of energetic fragment ions compa red to the conventional ion deflection technique, which has been used exten sively by other groups in the past. The higher energy resolution used in th e present experiments results in a complete separation of the thermal/quasi -thermal fragment ions from the energetic fragment ions. This, in turn, rev eals that the energetic fragment ions have a much narrower kinetic energy d istribution than previously assumed. Because of the comparatively weak ion signals, further studies such as separate threshold cross-section measureme nts and appearance energy determinations for, respectively, the "slow" and "fast" fragment ions could only be carried out for the CH3+ and C2H4+ fragm ent ions. (C) 2001 Elsevier Science B.V.