Cross sections for elastic scattering of electrons by CF₃Cl, CF₂Cl₂, and CFCl₃

Abstract

Differential, integral, and momentum transfer cross sections have been determined for the elastic scattering of electrons from the molecules CF₃Cl, CF₂Cl₂, and CFCl₃.With the help of a crossed electron beam-molecular beam apparatus using the relative flow technique, the ratios of the elastic differential cross sections (DCSs) of CF₃Cl, CF₂Cl₂, and CFCl₃ to those of He were measured in the energy region from 1.5 to 100 eV and at scattering angles in the range 15° to 130°. From those ratios, the absolute DCSs were determined by utilizing the known DCS of He. For CF₃Cl and CF₂Cl₂, at the common energies of measurement, we find generally good agreement with the results from the independent experiments of Mann and Linder [J. Phys. B 25, 1621 (1992); and ibid. 25, 1633 (1992)]. In addition, as a result of progressively substituting a Cl-atom, undulations in the angular distributions have been found to vary in a largely systematic manner in going from CF₄ to CF₃Cl to CF₂Cl₂ to CFCl₃ and to CCl₄. These observed features suggest that the elastic scattering process is, in an independently additive manner, dominated by the atomic-Cl atoms of the molecules. The present independent atom method calculation typically supports the experimental evidence, within the screened additivity rule formulation, for each species and for energies greater than about 10-20 eV. Integral elastic and momentum transfer cross sections were also derived from the measured DCSs, and are compared to the other available theoretical and experimental results. The elastic integral cross sections are also evaluated as a part of their contribution to the total cross section.