Electron Impact Total Ionization Cross Sections of CF4, C2F^6^ and C3F^8^

Published

October 12, 2021

Author(s)

H Nishimura, W M. Huo, Yong Sik Kim, M A. Ali

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. From the Fano plot, we have empirically deduced M²_ion. Which represents the leading dipole contribution at high T, to be 14.6, 23.2 and 35.1 for CF₄, C₂F₆ and C₃F₈, respectively. The molecular polarizability for C₃F₈ also was empirically estimated to be [alpha] equals 10.6 plus or minus 0.1 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 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 neutral dissociation, such as CF₄ gives CF₃ plus F, and hence is an upper limit to the experimental 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. We conclude that well-correlated wave functions will always lead to BEB cross sections higher than the experimental ionization cross sections because of the contributions from neutral dissociation. Our correlated wave functions are not fully optimized for C₂F₆ and C₃F₈ owing to the large number of bound electrons. Hence, we believe that for these two molecules the calculated cross sections are lower than the true BEB value, in spite of the apparent excellent agreement between the theory and the experiment.

Citation

Journal of Chemical Physics

Pub Type

Journals

Keywords

C₂F₆ and C₃F₈, neutral dissociation cross section, polarizability, total ionization cross section

Citation

Nishimura, H. , Huo, W. , Kim, Y. and Ali, M. (2021), Electron Impact Total Ionization Cross Sections of CF₄, C₂F^6^ and C₃F^8^, Journal of Chemical Physics (Accessed May 9, 2024)