The mobility of alkali ions in gases, I. The attachment of water molecules to alkali ions in gases

Abstract

Measurements of the mobility of gaseous ions carried out in the Wills Laboratory at Bristol have shown that a positive ion which at birth is a charged atom or molecule remains in its monomolecular state when the gas is pure. The relation between the mass of the ion and its mobility has been determined in a number of gases. In the presence of polar impurities each ion collects a cluster of molecules of impurity, so that it moves more slowly in an electric field. The purpose of this paper is to describe experiments made to study the formation of the clusters and the number of molecules they contain. Early experimenters measured the mobility of clustered ions, but thenvalues cannot be used for this purpose because the nature and concentration of the impurities present were unknown and uncontrollable. The only way of studying the problem is to use an apparatus made entirely of glass and metal, cleaned by a “ bake out ” and with tap grease and other contamination excluded. Controlled amounts of a known polar impurity can then be added to the pure gas. The experiments here described have been made at room temperature on the alkali positive ions in the inert gases with water vapour as the polar impurity.