Statistical–mechanical theory of gaseous ion–molecule reactions in an electrostatic field

Abstract

A theory is developed for gas‐phase swarm measurements of ion–molecule reactions in electrostatic fields of arbitrary strength. The theory allows measurements of reaction rate coefficients made at low temperatures and strong electric fields to be converted directly to equivalent thermal rate coefficients at elevated temperatures inaccessible by direct methods. It is not necessary to calculate the ion velocity distribution function explicitly, or to unfold the reaction cross section from the rate data. In first approximation the measured rate coefficient is equal to the thermal rate coefficient at an effective temperature calculated directly from the measured ion drift velocity. Higher approximations are obtained from more detailed analysis of the dependence of the rate coefficient and drift velocity on electric field strength. Comparison is made with experimental data reported in an accompanying paper by Albritton et al. In another accompanying paper, Lin and Bardsley compare the present theory with their detailed Monte Carlo calculations.