The Influence of the Townsend-Ramsauer Effect on the Electron Transport Coefficients of Molecular Gases

The influence of the Townsend-Ramsauer effect on the electron mobility, diffusion coefficient, mean energy and Townsend's alpha of hydrocarbon gases has been examined theoretically. In general it is shown that the Townsend-Ramsauer effect causes the mobility to decrease sharply with increase in electron mean energy giving rise to a drift velocity which is almost independent of the applied field; this plateau-like behaviour was experimentally observed by McGee and Jaeger in 1928. In the region in which the Townsend-Ramsauer effect prevails, the diffusion coefficient tends to remain constant and independent of the electron mean energy or the applied field. It is shown that, for gases exhibiting a Townsend-Ramsauer effect, the determination of reliable mean energy values as a function of E/p from swarm experiment data requires a knowledge of the total collision cross section and the electron energy distribution. The use of a Druyvesteyn distribution instead of a Maxwellian one appreciably reduces the mobility but hardly affects the diffusion coefficient. It is concluded from this and experimental evidence that the Maxwellian distribution does not hold for methane but may become more applicable as the molecular complexity of the hydrocarbons increases. The reasons for some of the discrepancies between theory and experiment are discussed.