Absolute cross sections for electron impact ionization and dissociative ionization of the SiF free radical.

Ionization by electron impact is one of the fundamental processes occurring in electrical discharges; free radicals are prominent species present in molecular gas discharges. Cross sections for ionization of free radicals, however, have rarely been measured. In this work a fast beam of the SiF free radical is prepared by charge transfer neutralization of an SiF sup + beam. Absolute ionization cross sections are measured for formation of the parent ion SiF sup + and the fragment ions Si sup + and F sup + from threshold to 200 eV. Approximately 30% of the Si sup + is found to form by ion pair production, that is, with F sup - as the other dissociation fragment.