Absolute total electron impact ionization cross-sections for perfluorinated hydrocarbons and small halocarbons

Absolute total positive-ion electron ionization cross-sections from threshold to 220 eV are reported for a range of halogenated methanes and small perfluorocarbons (2-4 carbon atoms). Correlations between the measured ionization cross-section and related molecular properties, in particular the vertical ionization potential (or vertical appearance energy) and molecular polarizability volume, are noted. Contributions to the total cross-section from individual bonds are also determined. Cross-sections predicted using these 'bond contributions' are in agreement with experiment for a wide range of molecules to better than +/- 10% accuracy, and in most cases to better than +/-5%. The experimental data are also compared with ionization efficiency curves calculated using the Deutsch-Mark (DM) and binary encounter Bethe (BEB) models.