Absolute cross sections for the dissociative electron impact ionization of the CF_x (x = 1-3) free radicals

We studied the dissociative ionization of the CF_x (x = 1-3) free radicals by electron impact and measured absolute partial cross sections and appearance energies for the formation of the various fragment ions. Fast beams (3-3.5 kV) of CF₃, CF₂ and CF were prepared by near-resonant charge transfer of CF⁺₃, CF⁺₂ and CF⁺ with Xe under conditions where the neutral beams contained primarily ground state ions with little vibrational excitation (0.5 eV or less). Absolute cross sections for the various molecular fragment ions were measured from threshold to 200 eV with peak cross sections of up to 1.3 Ų. The cross sections for the formation of the atomic C⁺ and F⁺ ions were found to be small (0.3 Ų or less) except for the F⁺ cross section from CF₂. The measured appearance energies indicate that the fragment ions are formed with excess kinetic energies ranging from essentially zero to about 3 eV per fragment. The CF⁺ cross section from CF₂ shows a second onset which corresponds to the double positive ion formation CF₂ → CF⁺ + F⁺. Extensive ion trajectory calculations were carried out in order to quantify the collection efficiency of our apparatus for fragment ions formed with excess kinetic energy.