Drift Velocities of Ions in Krypton and Xenon

Abstract

Drift velocities and mobilities of ions of Kr and Xe in their respective parent gases have been measured over a wide range of values of E/po, the ratio of electric field strength to normalized gas pressure. Two ions appear in each gas identified as KLr + and Kr2 + in Kr and Xe + and Xe2 + in Xe. The relation that drift velocity varies as (E/po)* at high E/po has been found to hold for the atomic ions and has been used to determine the equivalent hard sphere cross sections at high fields. The cross sections are 157X10 -16 cm 2 for Kr and 192 X 10~1 6 cm 2 for Xe. The Langevin theory of mobilities gives excellent agreement with experimental results extrapolated to zero field strength provided that, in the theory, the hard sphere cross section is taken as large for the atomic ions and very small for the molecular ions. The range of the polarization forces is such as to render them insignificant in atomic ion collisions and of primary importance in molecular ion collisions. T HE work reported by Hornbeck 1 on drift velocities in helium, neon, and argon has been extended using the same equipment to include krypton and xenon. The technique described in the references is essentially a time-of-flight method. As reported by him, an oscilloscope pattern is obtained in which sharp steps appear at time intervals after the start of the sweep equal to the transit time for ions to cross from anode to cathode in the parallel-plate Townsend tube.In Kr and in Xe, two steps appeared in the oscilloscope pattern for values of E/p 0 below 75 volts/ (cmXmm Hg). (The symbol p 0 is used to indicate that pressure readings are adjusted to the value which would be-produced by the same gas density at 0°C.) These two steps indicate the presence of two ions which are identified as the molecule ions Kr 2 + and Xe 2 + as the faster and the atomic ions Kr + and Xe + , respectively, as the slower ions. The identification is supported by the further observation that the supposed molecular ions disappear at higher E/p 0 in accordance with the greater difficulty of producing these ions, the fact that they do have greater speed (see reference 1), and the Kr + IN Kr .^y

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