Mass-Spectrometric Study of the Photoionization of C2F4 and CF4

Abstract

Photoionization‐efficiency curves for CF₃⁺ from CF₄ and for the parent ion and selected fragment ions from C₂F₄ are reported for the energy region from onset of ionization to about 20 eV. The ionization‐efficiency curves for both parent and fragment ions from C₂F₄ demonstrate a number of very pronounced autoionization peaks. Appearance potentials are made more reliable by using a cooled ionization chamber to remove most of the thermal vibrational energy of the molecules. From the difference between the appearance potentials of CF⁺ and CF₃⁺ from C₂F₄, it is concluded that I.P.(CF) = I.P.(CF₃) + 0.06 eV. Therefore the I.P.(CF) = 9.23 ± 0.08 eV can be calculated from the I.P.(CF₃) = 9.17 ± 0.08 eV derived in this paper. The onset of CF₃ from CF₄ is asymptotic, and there is no clear threshold. An upper limit for this threshold was set at 15.35 eV.