The theoretical analysis of the velocity distribution function of ions in weakly ionised gases is developed under the condition of induced dipole interaction between ion and molecule by using the BGK approximation in the Boltzmann equation. The derived functions is valid for arbitrary reduced field strength E/N and ion-neutral mass ratio m/M. The results are compared with the previous works of Whealton and Woo (1971), and Wannier (1958), and it is found that predicted distribution function exhibits great power in the case of nearly the same mass. Also the anisotropy of the distribution function is investigated.
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