Electron attachment to perfluorocarbon compounds. III. Fragmentation of aliphatic perfluorocarbons of interest to gaseous dielectrics

Resonance nondissociative and dissociative attachment of slow electrons (0-12 eV) to the perfluorocompounds 2-C₄F₆, c-C ₄F₆, 1,3-C₄F₆, c-C₄F ₈ and 2-C₄F₈ have been studied using a time-of-flight mass spectrometer. All five compounds capture thermal energy electrons and form long-lived parent negative ions with autodetachment lifetimes, τ_a, equal to 6, 7, 9, 6, and 10×10^-6 sec for c-C₄F₆^-*, 1,3-C₄F ₆^-*, 2-C₄F₆ ^-*, c-C₄F₈^-*, and 2-C₄F₈^-*, respectively. Except for 1,3-C₄F₆, the yield for the parent negative ions of these perfluorocarbons was more than a factor of 10 larger than that of the fragment ions. The following fragment negative ions were detected: 2-C₄F ₆: F^-, CF₃^-, C₃F ₃^-; c-C₄F₆: F^-, C ₃F₃^-, C₄F₅^-; 1,3-C₄F₆: F^-, CF₃^-, C₃F₃^-, C₄F₄^-; c-C₄F₈: F^-, CF₃^-, C ₂F₃^-, C₃F₅^-; and 2-C₄F₈: F^-, CF₃^-, C₂F₃^-, C₃F₃^-, C₃F₅^-*, C₄F₆ ^-*, C₄F₇^-*, (where the asterisk indicates that these fragment ions were found to be long-lived metastables with τ_a values equal to 70, 17, and 7×10 ^-6 sec, respectively). Groups of negative ion resonance peaks were found in the 0-2 eV and 4-6 eV electron impact energy range. Fragmentation patterns indicate that multiple decomposition proceeds via common negative ion states of C₄F_x^-* (x = 6 or 8). Fragment ions resulting from multiple bond cleavage and molecular rearrangement in the C₄F_x^-* systems were observed as well as ions resulting from direct cleavage. From the appearance onsets of some of the fragment ions we determined ΔH_f(C₃F₃ ^-) = -4.4 eV, ΔH_f(2-C₄F₆) = -9.8 eV, E.A. (C₂F₃) ≥ 1.6 eV, E.A. (C ₃F₅) ∼3.0 eV and other thermochemical data. The relevance of this work to the decomposition of dielectric gases is discussed.