Longitudinal and transverse diffusion coefficients for Li+ ion swarms in Kr gas.

Abstract

The ratio of the transverse diffusion coefficient to mobility, ${\mathit{D}}_{\mathit{T}}$/K at 309 K for ${\mathrm{Li}}^{+}$ ion swarms drifting in Kr gas in the E/N (electric field to neutral gas number density ratio) range of 5 to 170 Td, was experimentally determined with an overall accuracy of \ifmmode\pm\else\textpm\fi{}4%. The ${\mathit{D}}_{\mathit{T}}$/K results were effectively corrected for longitudinal end effects present appreciably in the drift tube by an analysis which requires the measurement of variance 〈${\mathit{x}}^{2}$〉 of the transverse ion-current density profile at different drift lengths z and the derivation of the magnitude ${\mathit{a}}_{2}$ of the end effects. Good agreement of the results with those calculated by Monte Carlo simulations (MCS) using an established interaction potential demonstrates the accuracy and reliability of the present ${\mathit{D}}_{\mathit{T}}$/K results. In addition, elaborate calculations of the reduced mobility ${\mathit{K}}_{0}$ and ratio ${\mathit{D}}_{\mathit{L}}$/K of the longitudinal diffusion coefficient to mobility of the ${\mathrm{Li}}^{+}$-Kr system are calculated with the MCS method. The accuracy of the MCS calculations is estimated to be \ifmmode\pm\else\textpm\fi{}2.5%. The calculated ${\mathit{D}}_{\mathit{L}}$/K values are compared with the experimental data available in the literature. Both ${\mathit{D}}_{\mathit{T}}$/K and ${\mathit{D}}_{\mathit{L}}$/K values for ${\mathrm{Li}}^{+}$ in Kr were also derived using reduced mobility ${\mathit{K}}_{0}$ data obtained from the MCS calculations and from experimental data, employing the generalized Einstein relations based on the three-temperature theory.