Blanc's Law—Ion Mobilities in Helium-Neon Mixtures

Abstract

The mobilities of ions of near thermal energies are measured in helium-neon mixtures using a drift velocity apparatus. These studies permit the investigation of ion motion in gases, e.g., ${\mathrm{He}}^{+}$ in Ne, under conditions where the charge transfer interaction is negligible compared to polarization attraction and short-range repulsion between ion and atom. In addition, the measurements provide a test of Blanc's empirical law, $\frac{1}{\mu }=\frac{f_1}{{\mu }_1}+\frac{f_2}{{\mu }_2}$, which relates the mobility $\mu$ in a binary mixture to the pure gas mobilities ${\mu }_1$ and ${\mu }_2$ and to the fractional gas concentrations $f_1$ and $f_2$. A theoretical treatment developed by Holstein is presented which shows that deviations from Blanc's law are limited to a few percent. The mobilities of ${\mathrm{He}}^{+}$, ${\mathrm{He}}_2^{+}$, and ${\mathrm{Ne}}_2^{+}$ ions are found to obey accurately Blanc's law. However, the "${\mathrm{Ne}}^{+}$" ion curve deviates markedly from the law. These deviations are explained in terms of the formation of moderately stable ${\mathrm{}\left(\mathrm{He}\mathrm{Ne}\right)\mathrm{}}^{+}$ ions from ${\mathrm{Ne}}^{+}$. Finally, the observed mobilities of ${\mathrm{He}}_2^{+}$ in Ne and ${\mathrm{Ne}}_2^{+}$ in He are found to agree with the predictions of polarization theory.