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

Abstract

Resonance nondissociative and dissociative attachment of slow electrons (0–12 eV) to the perfluorocompounds 2-C4F6, c-C4F6, 1,3-C4F6, c-C4F8, 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, τa, equal to 6, 7, 9, 6, and 10×10−6 sec for c-C4F6−*, 1,3-C4F6−*, 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-C4F6: F−, CF3−, C3F3−; c-C4F6: F−, C3F3−, C4F5−; 1,3-C4F6: F−, CF3−, C3F3−, C4F4−; c-C4F8: F−, CF3−, C2F3−, 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 τa 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 C4Fx−* (x = 6 or 8). Fragment ions resulting from multiple bond cleavage and molecular rearrangement in the C4Fx−* systems were observed as well as ions resulting from direct cleavage. From the appearance onsets of some of the fragment ions we determined ΔHf(C3F3−) = −4.4 eV, ΔHf(2-C4F6) = −9.8 eV, E.A. (C2F3) ? 1.6 eV, E.A. (C3F5) ∼3.0 eV and other thermochemical data. The relevance of this work to the decomposition of dielectric gases is discussed.