Very low energy electron scattering in C₆H₅F, C₆H₅Cl, C₆H₅Br and C₆H₅I

Integral and backward scattering cross sections are reported for the scattering of electrons from C₆H₅F, C₆H₅Cl, C₆H₅Br and C₆H₅I over the energy range between 10 and 20 meV to 10 eV. The measurements were made in transmission experiments using a synchrotron radiation photoionization apparatus with an energy resolution in the incident electron beam of approx. 3.5 meV (FWHM). The absolute accuracy of the cross sections is limited by experimental difficulties with respect to strong rotationally inelastic forward scattering, due to the dipolar nature of the target molecules. Integral scattering cross sections are found to rise to more than 1500 angstrom² at the lowest energies in C₆H₅F, C₆H₅Cl and C₆H₅Br and to more than 2000 angstrom² in C₆H₅I due to an additional contribution from electron attachment. At low energy, both integral and backward scattering cross sections are very similar in C₆H₅F, C₆H₅Cl and C₆H₅Br. Data are interpreted in terms of rotationally inelastic scattering, with values of cross sections essentially dictated by the correspondingly similar dipole moments (1.6-1.7 D) of these three species. The Born model for rotationally inelastic scattering consistently underestimates the experimental cross sections and overestimates the degree of forward scattering. Our data illustrate the limitations of the Born model for the estimation of low-energy scattering cross sections for plasma modelling.