Interpretation of calculated transverse and longitudinal diffusion for electrons in gases

Ratios of transverse $D_T$ and longitudinal $D_L$ diffusion coefficients to mobility $\mu$ and mean energies for electrons in gases are calculated for a wide range of $E/N$ for He, Ar, Xe, H$_2$, N$_2$, and CO. These transport coefficients are determined from spatial-gradient expansion, two-term spherical harmonic theory\footnote{J. H. Parker and J. J. Lowke, Phys. Rev. {\bf 181}, 290 (1969).}$^,$\footnote{G. J. M. Hagelaar (unpublished codes, 2012).} and from Monte Carlo simulations.\footnote{Ibid.}$^,$\footnote{S. F. Biagi, Nucl. Instr. and Meth. A {\bf 421}, 234 (1999).} As predicted by simplified theory\footnote{Parker, Lowke, Op. cit.} applied to the heavier rare gases, e.g., Ar and Xe, the ratio $D_T$/$D_L$ reaches 7 to 10 at mean electron energies for which the momentum transfer cross sections are rapidly rising functions of energy. Comparisons are made of simplified\footnote{Ibid.} and detailed predictions of $D_L$/$D_T$ values for N$_2$ and CO at low electron energies where the effects of scattering by the quadrupole potential of N$_2$ versus the dipole/quadrupole potential of CO are expected to be observed.