Comparison of Ion Chemistries in Octafluoro-2-butene ($2$-C$_{4}$F$_{8})$ and in Octfluorocyclobutane ($c$-C$_{4}$F$_{8})$*

$2$-C$_{4}$F$_{8}$ is one of the promising candidates to replace $c$-C$_{4}$F$_{8}$ that has been widely used for dielectric etching but is not environmentally friendly. In this study we have investigated electron impact ionization and ion-molecule reactions of $2$-C$_{4}$F$_{8}$ using Fourier transform mass spectrometry (FTMS), and compared the results with those of $c$-C$_{4}$F$_{8}$ we have studied previously. Electron impact ionization of $2$-C$_{4}$F$_{8}$ produces 15 ionic species including C$_{4}$F$_{7,8}^{+}$, C$_{3}$F$_{3,5,6}^{+}$, C$_{2}$F$_{4}^{+}$ and CF$_{1-3}^{+}$ as the major ions. The total ionization cross section of $2$-C$_{4}$F$_{8}$ reaches a maximum of 1.8x10$^{-15}$ cm$^{2}$ at 90 eV. The ionization is dominated by the channel forming the parent ion C$_{4}$F$_{8}^{+}$ from 12 to 18 eV, and by the channel forming C$_{3}$F$_{5}^{+}$ from 18 to 70 eV. After 70 eV, CF$_{3}^{+}$ becomes the dominant product ion. Among the major ions generated from the electron impact ionization of $2$-C$_{4}$F$_{8}$, only CF$^{+}$, CF$_{2}^{+}$ and CF$_{3}^{+}$ are found to react with $2$-C$_{4}$F$_{8}$, via F$^{-}$ abstraction or charge transfer mechanism. The charge transfer reaction of Ar$^{+}+2$-C$_{4}$F$_{8}$ produces primarily C$_{4}$F$_{7}^{+}$.