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, $\frac{1}{\ensuremath{\mu}}=\frac{{f}_{1}}{{\ensuremath{\mu}}_{1}}+\frac{{f}_{2}}{{\ensuremath{\mu}}_{2}}$, which relates the mobility $\ensuremath{\mu}$ in a binary mixture to the pure gas mobilities ${\ensuremath{\mu}}_{1}$ and ${\ensuremath{\mu}}_{2}$ 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 ${(\mathrm{He}\mathrm{Ne})}^{+}$ 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.