Electron drift and diffusion in polyatomic gases: Calculations for CH4, CD4, and related models

Abstract

We present the results of direct numerical calculations of electron drift velocities and diffusion coefficients in polyatomic gases as a function of applied electric field E. Model scattering cross sections appropriate to CH₄ and CD₄ and others of related form are used. The kinetic theory ideas presented in previous work are more extensively tested. We find good agreement with available experimental results for CH₄ and CD₄. The diffusion is found to be strongly anisotropic, consistent with the anisotropy of the (space‐independent) velocity distribution calculated previously. The diffusion parallel to the field has a strong maximum as a function of E, in a region where apparently no experimental work has been done. Drift and diffusion results for some CH₄ cross sections derived by comparison of experiment with approximate analytical treatments of the Boltzmann equation are discussed. We demonstrate that these treatments depend on parameters inadequate to specify the problem completely. The effects on drift velocity of angle dependence in the elastic scattering cross section are tested as are those of a resonance in the inelastic scattering. By comparison of our results for drift and diffusion with a proposed generalization of the Nernst–Einstein relation to large E, we show that this generalization is quantitatively invalid for gases with electron–molecule cross sections similar to those of CH₄ and CD₄.