Absolute partial cross sections for the parent ionization of the CF _x (x=1-3) free radicals by electron impact

We report absolute partial electron-impact ionization cross sections from threshold to 200 eV for the formation of the parent CF_x⁺ ions from the CF_x free radicals (x=1-3). Fast (3-3.5 kV) beams of CF₃, CF₂, and CF radicals were prepared by near-resonant charge transfer of CF₃⁺, CF₂⁺, and CF⁺ with triethylamine (TEA), Xe, and NF₃. The CF and CF₃ neutral beams were found to contain primarily ground state neutral radicals with some evidence for the presence of a small amount of vibrational excitation (0.5 eV or less). Contributions to the CF ₂⁺ ion signal obtained at electron energies several electron volts below the 11.4 eV threshold for the ionization of ground state CF₂ radicals indicate the presence of CF₂ metastables (presumably in the ³B₁ state) in the CF₂ neutral beam. The level of CF₂ metastable contamination was found to depend critically on the exact experimental conditions. At 70 eV, the absolute parent ionization cross sections are 0.38±0.07 Ų (CF₃→CF₃⁺), 1.03±0.16 Ų (CF₂→CF₂⁺), and 1.25±0.19 Ų (CF→CF⁺), respectively.