A time-resolved study of ionization, electron attachment and positive-ion drift in methane

A pulsed Townsend swarm technique has been used to study electron-ionization and electron-attachment processes in methane over the density-normalized electrical field strength, E/N, in the range 1.2-30 fV cm2 (120-3000 Td). The time-resolved ionic avalanches were obtained under conditions such that the occurrence of electron-attachment processes was detected unambiguously. The analysis of these avalanche pulses resulted in values of the effective ionization coefficients and positive-ion mobilities over the above E/N range. To the best of our knowledge, no values of positive-ion mobilities had been published before for E/N>560 Td. The electron-attachment effects were observed up to E/N=320 Td. (author)