Threshold electron impact ionization of carbon tetrafluoride, trifluoromethane, methane and propane

We have studied the ionization behaviour close to threshold for the fragment ion production from CF4, CHF3, CH4 and C3H8. In particular, we use a reliable method to extrapolate weak ion signals towards the appearance energy. Our evaluation is based on an extension of Wannier's law for the ionization of atoms towards the case of polyatomic molecules. In the experiment we use a hemispherical electron monochromator with an energy resolution of ≈135 meV. The vibrationally cold target molecules are prepared in a supersonic expansion and the resulting ions are mass analysed using a quadrupole mass spectrometer. We observe several disagreements with recommended ionization energy values, reaching up to 5 eV in the CHF3 case. This is in contrast to a number of recent reliability tests that we have performed on Ne, Ar, Kr, Xe, H2, D2, N2, O2 and N2O and that have reproduced the tabulated literature data within the resolution of the monochromator.