Electron drift velocities in molecular-gas-rare-gas mixtures

The drift velocity of electrons in mixtures containing a molecular gas in a rare-gas diluent has been calculated as a function of $\frac{E}{N}$ (ratio of the electric field to neutral number density) and of molecular-gas concentration using a numerical solution of the Boltzmann transport equation. The results agree well with available experimental data where gas purity was strictly controlled. In argon, it is shown that very small amounts of a molecular additive can drastically alter the behavior of the electron drift velocity. The curves exhibit a local maximum with respect to $\frac{E}{N}$ and an absolute maximum with respect to molecular concentration. A useful formula for the mobility has been derived, which explains the peculiar behavior of the electron drift velocity in these gas mixtures.