Mobility, diffusion, and clustering of K+ ions in gases

We have measured, with a drift tube mass spectrometer, the mobilities and longitudinal diffusion coefficients of K⁺ ions in nitrogen and carbon monoxide at 300°K. The measurements were made over a range of E/N extending from thermal values up to ${\text{636 × 10}}^{\text{−17}}\hspace{0.17em}\mathrm{V}·{\mathrm{cm}}^2$ . Here E is the drift field intensity and N is the gas number density. The zero‐field reduced mobilities of K⁺ ions in N₂ and CO were determined to be $\text{(2.54 ± 0.05)}$ and $\text{(2.30 ± 0.04)\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V}·\sec$ , respectively. The low‐field diffusion coefficients are in excellent agreement with the values calculated by the Einstein equation from the experimental zero‐field mobilities. The experimental diffusion coefficients are compared with the predictions of an equation developed by Wannier on the assumption that the ion‐molecule interaction consists of only the attractive polarization force, of which a constant mean free time between collisions is a consequence. Comparison is also made with a modified version of this equation which contains the ionic drift velocity rather than the mean free time. The agreement between the experimental data and the modified Wannier equation is very close up to average ionic energies of about 0.6 eV. We also observed clustering of single molecules of CO₂, NO, CO, N₂, O₂, Ar, D₂, Ne, and He to K⁺ ions when the drift tube was filled successively with the indicated gases. At room temperature, clustering is likely to be of importance in studies of the transport properties of K⁺ ions in all of these gases except D₂, Ne, and He.