Electron impact ionization cross sections of the SiF sub 3 free radical.

Absolute cross sections for electron-impact ionization of the SiF sub 3 free radical from threshold to 200 eV are presented for formation of the parent SiF sub 3 sup + ion and the fragment SiF sub 2 sup +, SiF sup +, and Si sup + ions. A 3 keV beam of SiF sub 3 is prepared by near-resonant charge transfer of SiF sub 3 sup + with 1,3,5-trimethylbenzene. The beam contains only ground electronic state neutral radicals, but with as much as 1.5 ev of vibrational energy. The absolute cross section for formation of the parent ion at 70 eV is 0.67+-0.09angstrom sup 2. At 70 eV the formation of SiF sub 2 sup + is the major process, having a cross section 2.51+-0.02 times larger than that of the parent ion, while the SiF sup + fragment has a cross section 1.47+-0.08 times as large as the parent. Threshold measurements show that ion pair dissociation processes make a significant contribution to the formation of positively charged fragment ions.