Rate coefficients of ion-molecule processes in gases under varying electric field and ion density conditions

The perturbation theory developed previously for describing electron swarm properties is used to describe ion-molecule rate coefficients in a gas and weakly ionized plasma under non-uniform electric field and ion-density conditions when the deviation from spatial uniformity is small. The effect of slow variation in time of the electric field is also considered. The space- or time-dependent rate coefficients are represented as a sum of the local values and the product of new rate coefficients dependent on the local field and the gradients (or time derivative) of the field and ion density. The new rate coefficients are determined for an ion motion in its parent gas under the model assumptions of the energy dependence of the cross sections for ion-molecule reactions. It is shown that the relaxation in space or time of high-energy ions is much slower than the relaxation of low-energy ions. The effect is more pronounced for ions in a non-uniform electric field. The suitability of the new rate coefficients is demonstrated for estimating the role of non-local effects in ion swarm experiments and low current discharges and analysing the development of attachment instability in the plasma of electronegative gases.