New electric-field scaling law for swarm experiments

We generalize the familiar result that the drift velocity Vd and the product pD of the pressure and the diffusion coefficient depend on pressure p and electric field E only through the ratio E/p in swarm experiments. Our new scaling law applies to different systems to the extent that their elastic and inelastic cross sections are in the same ratio. We present model calculations of Vd for electrons in gaseous CH₄-Ar and CH₄-⁴He mixtures predicting that the scaling is approximately obeyed if the CH₄ concentrations are appropriately related. We also examine Vd and D_⊥ data for electrons in CH₄ and SiH₄ at low E and find that the scaling is approximately obeyed with a factor of 6. This suggests strongly that the low energy ({}1 eV) effective elastic and inelastic electron cross sections for these molecules have approximately the same shape and differ by the same factor, which is approximately 6. Thus in particular we predict a Ramsauer minimum for SiH₄, which has been suggested previously.