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

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., 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, 1μ=f₁μ₁+f₂μ₂, which relates the mobility μ in a binary mixture to the pure gas mobilities μ₁ and μ₂ and to the fractional gas concentrations f₁ and f₂. 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 He⁺, He₂⁺, and Ne₂⁺ ions are found to obey accurately Blanc's law. However, the "Ne⁺" ion curve deviates markedly from the law. These deviations are explained in terms of the formation of moderately stable (HeNe)⁺ ions from Ne⁺. Finally, the observed mobilities of He₂⁺ in Ne and Ne₂⁺ in He are found to agree with the predictions of polarization theory.