On an Anomaly in the Mobility of Gaseous Ions

The first semiquantitative understanding of the motion of gaseous ions in electric fields was achieved by Langevin. 1 He adopted as a model force between the ions and the gas molecules a superposition of the attractive polarization force and a hard core repulsion. He then applied kinetic theory to the mixture of ions and molecules and determined the response of the ions in such a mixture to a small field. A drift velocity proportional to the field was the result. The constant of proportionality is called the mobility. Langevin produced the first estimates for this number. There has been no essential departure from Langevin's approach in subsequent years, but only refinements and extensions; they occurred generally in close correlation with experiment. 2 - 5 A useful extension was the one to high fields. One gets then a drift velocity versus field curve rather than a simple constant of proportionality. In favorable cases, the analysis of such data has been carried out in a quite satisfactory way.1" The general rule is that if the results are expressed in terms of a mobility, then the mobility tends to decrease with increasing * Mr. Wannier is Professor of Physics, University of Oregon, Eugene. He performed this work while on sabbatical leave at Bell Telephone Laboratories, Murray Hill, New Jersey. t See Refs. 6 and 7. In Ref. 6, the abscissas on Figs. 3-7 are a power of ten too small. 343