STUDIES OF TRANSVERSE DIFFUSION OF ALKALI IONS IN NOBLE GASES

Abstract

A new drift-tube apparatus has been developed for measuring DT/K, the ratio of the transverse diffusion coefficient to the mobility of ions in a neutral gas under the influence of an electric field. The main features are the ion current detector, which comprises 65 parallel rods, and an external mechanical multiplexer, which was designed to have extremely low (fA order) leakage currents and noise levels. Unlike with other earlier systems, the need for precise mechanical motion or adjustments of experimental conditions inside the drift tube during data collection has been circumvented. Its design therefore simplifies construction and makes operation less error-prone. Data collection is computer controlled. A value for DT/K can now typically be obtained in less than 20 minutes, which greatly increases the rate at which results can be obtained as well as reduces the chance of unwanted variations in experimental conditions occurring during data collection. Results are estimated to be accurate to within 3%. Room temperature measurements of DT/K were made for Na+ ions in neon and argon, and K+ ions in helium, argon and krypton. Values were obtained at values of E/N, the ratio of the drift tube electric field to neutral gas number density, ranging from 10 to 200 Td for Na+ in neon, 5 to 260 Td for Na+ in argon, 10 to 220 Td for K+ in helium, 10 to 600 Td for K+ in argon, and 5 to 420 Td for K+ in krypton. As a case study of one use of measured values of DT/K, the experimental values are compared with values obtained from Monte Carlo simulations using several proposed interaction potentials as input. The comparisons suggest that none of the potentials examined is a totally accurate representation of the K+-Ar interaction potential.