Electron-impact total ionization cross sections of CF4, C2F6, and C3F8

Both theoretical and experimental electron-impact total ionization cross se ctions of CF4, C2F6, and C3F8 are presented. The experimental cross section s have been measured as a function of incident electron energy T from thres hold to 3 keV. A parallel plate condenser type apparatus was used. The mole cular polarizability for C3F8 was empirically estimated to be alpha = 10.6 Angstrom(3) +/- 0.8 Angstrom(3). Theoretical cross sections calculated from the binary-encounter-Bethe (BEB) method, which combines a modified form of the Mott cross section and the Bethe cross section, are compared with the experimental cross sections. The BEB cross sections calculated from correla ted molecular wave functions with theoretical estimates for multiple ioniza tion are about 10% higher than the experimental data at the peak for CF4, w hile they are in excellent agreement with the experimental data for C2F6 an d C3F8. Our analysis shows that the BEB theory implicitly includes part of neutral dissociation, such as CF4 --> CF3 + F, and hence tends to be an upp er limit to the total ionization cross section. We found that the differenc e between our best theory for CF4 and the present experimental cross sectio n exhibits a remarkable similarity to the shape of the recently measured cr oss section for neutral dissociation, though there is no a priori reason fo r the similarity. Owing to the large number of bound electrons, the correla tion included in our wave functions for C2F6 and C3F8 is more limited than for CF4. Hence, we believe that for these two molecules the calculated cros s sections are lower than the true BEB values, in spite of the apparent exc ellent agreement between the theory and the experiment. (C) 1999 American I nstitute of Physics. [S0021-9606(99)01608-6].