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

摘要

Resonance nondissociative and dissociative attachment of slow electrons (0ndash;12 eV) to the perfluorocompounds 2hyphen;C4F6,chyphen;C4F6, 1,3hyphen;C4F6,chyphen;C4F8, and 2hyphen;C4F8have been studied using a timehyphen;ofhyphen;flight mass spectrometer. All five compounds capture thermal energy electrons and form longhyphen;lived parent negative ions with autodetachment lifetimes, tgr;a, equal to 6, 7, 9, 6, and 10times;10minus;6sec forchyphen;C4F6minus;ast;, 1,3hyphen;C4F6minus;ast;, 2hyphen;C4F6minus;ast;,chyphen;C4F8minus;ast;, and 2hyphen;C4F8minus;ast;, respectively. Except for 1,3hyphen;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: 2hyphen;C4F6: Fminus;, CF3minus;, C3F3minus;;chyphen;C4F6: Fminus;, C3F3minus;, C4F5minus;; 1,3hyphen;C4F6: Fminus;, CF3minus;, C3F3minus;, C4F4minus;;chyphen;C4F8: Fminus;, CF3minus;, C2F3minus;, C3F5minus;; and 2hyphen;C4F8: Fminus;, CF3minus;, C2F3minus;, C3F3minus;, C3F5minus;ast;, C4F6minus;ast;, C4F7minus;ast;, (where the asterisk indicates that these fragment ions were found to be longhyphen;lived metastables with tgr;avalues equal to 70, 17, and 7times;10minus;6sec, respectively). Groups of negative ion resonance peaks were found in the 0ndash;2 eV and 4ndash;6 eV electron impact energy range. Fragmentation patterns indicate that multiple decomposition proceeds via common negative ion states of C4Fxminus;ast; (xthinsp;=thinsp;6 or 8). Fragment ions resulting from multiple bond cleavage and molecular rearrangement in the C4Fxminus;ast; systems were observed as well as ions resulting from direct cleavage. From the appearance onsets of some of the fragment ions we determined Dgr;Hf(C3F3minus;) = minus;4.4 eV, Dgr;Hf(2hyphen;C4F6) = minus;9.8 eV, E.A. (C2F3) quest;thinsp;1.6 eV, E.A. (C3F5) sim;3.0 eV and other thermochemical data. The relevance of this work to the decomposition of dielectric gases is discussed.