Ion mobility and liquid motion in liquefied argon

Abstract

Current-voltage characteristics and ion mobilities have been determined for liquefied argon of reasonable purity at fields up to 10 kv cm⁻¹ using a triode cell with a radioactive emitter. The current was space-charge limited up to about 1 kv cm⁻¹, but at higher fields there was only a partial saturation, indicating the influence of current-induced liquid motion. Evidence that the grid electrode of the triode cell produced a strong blocking action to ion neutralization was also found. The current transients obtained by switching the grid potential provided values of the mobility of Ar₂⁺ and O₂⁻ ions in the liquid and also offered evidence of free electrons and electron attachment to form the O₂⁻ ion. The mobility of both ions was influenced by self-induced liquid motion. Although the lowest measured mobility was about 36×10⁻⁴cm²v⁻¹s⁻¹, extrapolation to zero current suggested a true ion mobility, uninfluenced by liquid motion, of about 2×10⁻⁴cm²v⁻¹s⁻¹. Differences between values reported earlier, which are significantly larger than this value, are probably due to liquid motion. The velocity of both ions at higher values of cell current varied directly with the square root of the drift field, the combined effects of local polarization about the ion and induced liquid motion probably being responsible.