Pressure-dependent negative ion mobilities in H$_{2}$O and its mixtures with Ar and N$_{2}$

We have used a pulsed Townsend technique to measure the mobility of negative ions of H$_{2}$O in parent gas and in the mixtures of H$_{2}$O with N$_{2}$ and Ar. The E/N range of measurement was 10-40 Td, while the partial pressure of H$_{2}$O was varied up to 18 torr. For a mixture of given composition, we have observed a well defined dependence of the mobility of the ionic species with total gas pressure. Besides, we have found that the measurement of the mobilities at a single pressure and several mixture ratios follow Blanc's law closely. These findings are indicative of negative-ion cluster formation, the sizes of which increase with the total pressure of the mixture. The identity of the drifting ions has not yet been assessed.