Mobilities of Mass-IdentifiedH3+andH+Ions in Hydrogen

The drift velocities of mass-identified $\mathrm{H}_3^{}{}_{}{}^{+}$ and ${\mathrm{H}}^{+}$ ions in hydrogen gas at room temperature were measured. The $\mathrm{H}_3^{}{}_{}{}^{+}$ ions were found to be in thermal equilibrium with the gas at $\frac{E}{p_0}$ less than about 10 V/cm Torr; the ${\mathrm{H}}^{+}$ ions, at $\frac{E}{p_0}$ less than about 5 V/cm Torr. From these measurements, the reduced zero-field mobilities were deduced: $\mathrm{H}_3^{}{}_{}{}^{+}$, 11.1±0.6 ${\mathrm{cm}}^2$/V sec; ${\mathrm{H}}^{+}$, 16.0±0.8 ${\mathrm{cm}}^2$/V sec. This investigation was performed with a long, low-pressure drift tube using a pulsed time-of-flight technique. The arrival-time histograms presented evidence of hydrogen ion-molecule reactions. It is shown that these reactions introduce no ambiguity in ascribing the above zero-field mobilities to single ionic species. Only a negligible fraction of the detected $\mathrm{H}_3^{}{}_{}{}^{+}$ ions were formed by the three-body conversion of ${\mathrm{H}}^{+}$ into $\mathrm{H}_3^{}{}_{}{}^{+}$. Above an $\frac{E}{p_0}$ of about 54 V/cm Torr, the disruption of $\mathrm{H}_3^{}{}_{}{}^{+}$ ions contributes substantially to the ${\mathrm{H}}^{+}$ signal. The reactive formation of $\mathrm{H}_5^{}{}_{}{}^{+}$ from $\mathrm{H}_3^{}{}_{}{}^{+}$ was evident in the $\mathrm{H}_5^{}{}_{}{}^{+}$ arrival-time histograms. The zero-field mobility of potassium ions in hydrogen was also determined, and the close agreement with the data of other investigators demonstrates that the apparatus is relatively free of unknown systematic uncertainties.