Electron-Impact Total Ionization Cross Sections of CF4, C2F6, and C3F8

Published

February 1, 1999

Author(s)

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

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 appartus was used. The molecular polarizability for C₃F₈ was empirically estimated to be α = 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 crosws sections. The BEB cross sections calculated from correlated molecular wave functions with theoretical estimates for multiplee ionization are about 10% higher than the experimental data at 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, through 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 ₂F₆ and the large number of bound electrons, the correlation included in our wave functions for C₂F₆andd 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.

Citation

Journal of Chemical Physics

Volume

110

Issue

No. 8

Pub Type

Journals

Keywords

electron-impact total ionization

Citation

Nishimura, H. , Huo, W. , Ali, M. and Kim, Y. (1999), Electron-Impact Total Ionization Cross Sections of CF₄, C₂F₆, and C₃F₈, Journal of Chemical Physics (Accessed December 26, 2024)