Mobility of negative ions of H2O in H2O, Ar and N2

We have measured the mobility of the negative ions of H2O in the H2O, H2O-Ar and H2O-N2 mixtures using a pulsed Townsend technique. We have found that a single or predominant negative ion with a mobility of 0.68 cm2V-1s-1 drifts in H2O over the pressure range 10-16 torr, and this has been inferred to be the cluster species OH-(H2O)2. Regarding the H2O-Ar mixtures we found that the negative ion mobility is pressure-dependent, suggesting an increase in the mass of the majority ion with pressure; moreover, a test of these mobilities (low-field) with Blanc’s law indicates that for a given pressure there exists a predominant species. A similar though less marked behaviour is observed for the H2O-N2 mixtures. 1