Drift velocities of electrons in methane-inert-gas mixtures

The drift of electrons in mixtures of methane with argon and helium is measured with a double shutter drift tube as a function of methane composition and electric field-pressure ratio E/p. At certain concentrations, inelastic scattering by methane causes a maximum in the drift velocity as a function of E/p. As the methane mole fraction decreases, the drift velocity maximum decreases and moves to lower values of E/p. In the argon mixtures, the drift velocity at low E/p is greater than it is in either pure gas. Comparision is made with direct numerical solutions of the Boltzmann equation for the mixtures. When the methane mole fractions are appropriately chosen, the data obey roughly a scaling law relating the electron drift velocity versus E/p in a methane-argon mixture with VD versus E/p in a methane-helium mixture.