Mobility of Positive Ions in Liquefied Argon and Nitrogen

Drift velocities of positive ions in liquid argon and liquid nitrogen have been measured for applied electric fields in the range from 0 to 4300 V/cm. These data were taken at pressures of 1 atm and at temperatures corresponding to the respective liquids' boiling points at this pressure. A time-of-flight spectrometer, consisting of an ion source, an electronic shutter, and a final drift space, was used. The ions were produced by the technique of field ionization. This was accomplished by immersing a tungsten point (etched down to a radius of less than 1000 Å) in the respective liquefied gases and applying a high potential to it. The times of flight of the ions across the final drift space were determined by amplifying the ion current and displaying it on an oscilloscope. It was found that step-like changes in the curves of ion mobility versus $E$ occurred in both liquid argon and liquid nitrogen. Five such constant-mobility regions were found in liquid argon and four in liquid nitrogen. These constant mobilities were found to be 6.0×${10}^{-4\mathrm{}}$, 9.75×${10}^{-4\mathrm{}}$, 8.50×${10}^{-4\mathrm{}}$, 7.75×${10}^{-4\mathrm{}}$, and 7.25×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^2$/V-sec in liquid argon, and 2.50×${10}^{-3\mathrm{}}$, 1.80×${10}^{-3\mathrm{}}$, 1.54×${10}^{-3\mathrm{}}$, and 1.36×${10}^{-3\mathrm{}}$ ${\mathrm{cm}}^2$/V-sec in liquid nitrogen. It is suggested that these mobilities may correspond to ionic clusters.