ELECTRON-IMPACT IONIZATION OF THE NF3 MOLECULE

We report measurements of the absolute cross sections for the electron-impact ionization of the NF3 molecule from threshold to 200 eV. Absolute values and shapes for the NF3 parent ionization cross section were obtained independently in two different laboratories using two different experimental techniques. The agreement was better than +/-8%. At 70 eV, the absolute parent NF3 ionization cross section is 0.35 +/- 0.06 angstrom2. The dissociative ionization of NF3 was studied primarily using the fast-beam technique. Absolute cross sections for the formation of the fragment ions NF2+ and NF+ were found to be 1.05 +/- 0.20 angstrom2 (NF2+) and 0.70 +/- 0.15 angstrom2 (NF+) at 70 eV. Two channels contribute to the observed NF+ ion signal, the single positive ion formation NF3 --> NF+ + 2F and the positive ion pair formation NF3 --> NF+ + F+ + F with an appearance energy of 36.5 +/- 1.O0 eV. While the NF2+ fragment ions are formed with little excess kinetic energy for impact energies near threshold, the NF+ fragment ions are formed with a broad distribution of excess kinetic energies ranging from essentially zero to about 4 eV. Cross sections for the formation of the atomic fragment ions F+ and N+ were found to be small with a maximum value of 0.3 angstrom2 for F+ (at 70 eV) which can be attributed primarily to the double positive ion formation NF3 --> NF+ + F+ + F. The upper limit for the N+ cross section from NF3 is 0.1 angstrom2 at 70 eV.