ABSOLUTE PARTIAL CROSS-SECTIONS FOR THE PARENT IONIZATION OF THE CFX (X=1-3) FREE-RADICALS BY ELECTRON-IMPACT

We report absolute partial electron-impact ionization cross sections from threshold to 200 eV for the formation of the parent CF(x+) ions from the CF(x) free radicals (x = 1-3 ). Fast (3-3.5 kV) beams of CF3, CF2, and CF radicals were prepared by near-resonant charge transfer of CF3+, CF2+, and CF+ with triethylamine (TEA), Xe, and NF3. The CF and CF3 neutral beams were found to contain primarily ground state neutral radicals with some evidence for the presence of a small amount of vibrational excitation (0.5 eV or less). Contributions to the CF2+ ion signal obtained at electron energies several electron volts below the 11.4 eV threshold for the ionization of ground state CF2 radicals indicate the presence of CF2 metastables (presumably in the B-3(1) state) in the CF2 neutral beam. The level of CF2 metastable contamination was found to depend critically on the exact experimental conditions. At 70 eV, the absolute parent ionization cross sections are 0.38 +/- 0.07 angstrom2 (CF3 --> CF3+), 1.03 +/- 0.16 angstrom2 (CF2 --> CF2+), and 1.25 +/- 0.19 angstrom2 (CF --> CF+), respectively.