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

Abstract

Both theoretical and experimental electron-impact total ionization cross sections of CF4, C2F6, and C3F8 are presented. The experimental cross sections have been measured as a function of incident electron energy T from threshold to 3 keV. A parallel plate condenser type apparatus was used. The molecular polarizability for C3F8 was empirically estimated to be α=10.6 Å3±0.8 Å3. 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 correlated molecular wave functions with theoretical estimates for multiple ionization are about 10% higher than the experimental data at the peak for CF4, while they are in excellent agreement with the experimental data for C2F6 and 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 upper limit to the total ionization cross section. We found that the difference between our best theory for CF4 and the present experimental cross section exhibits a remarkable similarity to the shape of the recently measured cross section for neutral dissociation, though there is no a priori reason for the similarity. Owing to the large number of bound electrons, the correlation 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 cross sections are lower than the true BEB values, in spite of the apparent excellent agreement between the theory and the experiment.