Temperature Dependence of Helium-Ion Mobilities

The mobilities of the ${\mathrm{He}}^{+}$ and $\mathrm{He}_2^{}{}_{}{}^{+}$ ions in helium were measured as a function of $\frac{E}{n}$ and gas temperature for $T=300-76\mathrm{}$ °K. At temperatures below 200 °K and at very low $\frac{E}{n}$, conversion of the $\mathrm{He}_2^{}{}_{}{}^{+}$ to $\mathrm{He}_3^{}{}_{}{}^{+}$ was observed to take place. Mobilities of $\mathrm{He}_3^{}{}_{}{}^{+}$ were determined at 115 and 76 °K. The zero-field mobility of ${\mathrm{He}}^{+}$ was found to increase with decreasing temperature and was in excellent agreement with the theoretical predictions of Dickinson. Both the $\mathrm{He}_2^{}{}_{}{}^{+}$ and $\mathrm{He}_3^{}{}_{}{}^{+}$ mobilities decreased with decreasing temperature. The $\mathrm{He}_2^{}{}_{}{}^{+}$ mobilities were all significantly lower than the Langevin polarization limit, and were lower even than the mobilities of the heavier $\mathrm{He}_3^{}{}_{}{}^{+}$ ion.