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

Electron impact ionization cross sections have been measured for the halogen atoms F, Cl, Br and l from threshold to 200 eV with an absolute accuracy of +- 14%. The halogen cross sections have shapes similar to those of the neighboring rare gases but are greater in magnitude. Simple semi-empirical and semi-classical formulae of electron impact ionization are able to accurately predict the experimentally determined halogen- to-rare gas cross section ratios.