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.

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