Mobility of positive alkali ions in argon, neon and helium

In a previous paper we described the application of new methods of measuring the mobility of ions to the case of positive ions of helium moving through helium gas. We also gave an example of the profound effect of traces of impurities in giving rise to groups of ions moving at speeds different from that of helium ions. This result is contrary to those obtained in many earlier investigations on the mobility of ions produced in one gas and measured in another. It was found that such ions could not be distinguished in their mobility from an ion of the gas in which the measurements were made. This is explained by the fact that, judged by present standards, the gas in all these early experiments was very impure and the ions consisted of clusters of polar molecules. Moreover, except in the case of the radio-active recoil atoms investigated by Rutherford and by Franck, which have a low ionisation potential, the conditions were often favourable for a transfer of charge from the original ion to some other atom or molecule, so that even the core of the cluster may not have been that of the original ion. In the determination of the mobility of different ions in a gas, the positive ions may be produced from Kunsman sources of the alkalies or alkaline earths, or from a glow discharge in a gas containing small amounts of known impurities. The present paper describes the results obtained in argon, neon and helium by the first method using ions of sodium, potassium, rubidium and cæsium.