Absolute electron-impact-ionization cross-section measurements of the halogen atoms

Electron-impact-ionization cross sections have been measured for the halogen atoms F, Cl, Br, and I from threshold to 200 eV. The absolute accuracy is ±{}14%, except for F which is ±{}20%. The halogen cross sections have shapes similar to those of the neighboring rare gases but are greater in magnitude. Simple semiclassical formulas of electron-impact ionization are able to accurately predict the experimentally determined halogen to rare gas cross-section ratios.