Drift and clustering of daughter negative ions of H2O in parent gas

Abstract

The mobility of daughter negative ions of H2O in parent gas has been measured with a pulsed Townsend technique over the density-reduced field strength, E/N, range 9–100 Td and a pressure range 2–16 Torr. It has been found that the mobility of the anions is dependent on the gas pressure. Using a transport theory considering the influence of the permanent dipole field of H2O, we have found that the pressure-dependent mobilities can be associated with a series of cluster ions of the type OH−(H2O)n (n = 1–3), with the mass of the cluster species increasing with the total gas pressure. Also, the mobility of H− and OH− could be estimated. Using a Townsend avalanche simulator we have been able to explain the measured ionic currents in terms of an ion–molecule reaction scheme with a single set of swarm and reaction coefficients for each value of the density-reduced field strength, E/N, at several pressures. Regarding the positive ions, the only drifting ion is H3O+, the mobility of which could be estimated. The rate constants relative to the formation of the OH−(H2O)n (n = 1–3) species were also derived from this study.