Electron impact ionization of the SF5 and SF3 free radicals

We measured absolute cross sections for the electron-impact ionization and dissociative ionization of the SF5 and SF3 free radicals from threshold to 200 eV using the fast-neutral-beam technique. The total single ionization cross sections at 70 eV were found to be 5.1 x 10(-16) cm(2) (SF5) and 3.2 x 10(-16) cm(2) (SF3). The cross sections reach their maximum at about 100 eV for both radicals, with values of 5.4 x 10(-16) cm(2) (SF5) and 3.5 x 10(-16) cm(2) (SF3). Dissociative ionization is important only for SF5, where the SF5+ parent ionization cross section and the SF4+ fragment ionization cross sections have roughly the same value. By contrast, the ionization of SF3 is dominated by the formation of SF3+ parent ions. A comparison of the measured total single ionization cross sections with calculated cross sections, using a modified additivity rule, shows excellent agreement for both radicals in terms of the absolute cross section values and the cross section shapes. Total single ionization cross sections were also calculated for SF4, SF2, and SF. (C) 1998 American Institute of Physics. [S0021-9606(98)00940-3].