Electron-velocity distribution functions in gases: The influence of anisotropic scattering and electron nonconservation by attachment and ionization

Electron-velocity distribution functions have been calculated for electrons moving in a gas under the influence of an electric field. By assuming a differential scattering cross section such that scattering is entirely at 00 and 1800, it is shown analytically that the electron drift velocity w, the mean electron energy , the transverse diffusion coefficient D/sub T/, and the longitudinal diffusion coefficient D/sub L/ are not, in general, determined by the momentum-transfer cross section alone. The magnitude of the errors incurred by neglecting the higher-order terms in the spherical-harmonic expansion of the velocity distribution function for this collision model are indicated. Velocity distribution functions have also been calculated for attaching gases. It is found that the velocity distribution function can be considerably altered by attachment for pure gases and gas mixtures. Consequently, any attempt to determine collision cross sections from electron-swarm measurements of w, D/sub T/, and D/sub L/ in attaching gases should include attachment processes in the solution of the Boltzmann equation. The magnitude of this dependence has been found to be determined largely by the ratio of the attachment collision frequency to the energy-transfer collision frequency and the velocity dependences of the momentum transfer and attaching collision frequencies