Positive-Ion Mobilities in Dry Air

Drift velocities of mass-identified positive ions in air have been measured to about ±5% as a function of $\frac{E}{N}$ for $2\times {10}^{-16\mathrm{}}\lesssim \frac{E}{N}\lesssim 2\times {10}^{-14\mathrm{}}$ V ${\mathrm{cm}}^2$ and for $1.8\times {10}^{15}\lesssim N\lesssim 7\times {10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$. These measurements lead to values of the zero-field mobilities of 1.6, 3.5, and 2.5 ${\mathrm{cm}}^2$/V sec for $\mathrm{N}_2^{}{}_{}{}^{+}$, N${\mathrm{O}}^{+}$, and $\mathrm{O}_2^{}{}_{}{}^{+}$, respectively, in air. The high-$\frac{E}{N}$ data yield momentum-transfer cross sections of 110, 21, and 30 ${\mathrm{\AA }}^2$ for $\mathrm{N}_2^{}{}_{}{}^{+}$, N${\mathrm{O}}^{+}$, and $\mathrm{O}_2^{}{}_{}{}^{+}$, respectively. The rapid disappearance of ${\mathrm{N}}^{+}$ and ${\mathrm{O}}^{+}$ due to ion-molecule reactions prevented good drift-velocity measurements for them, but partial results indicate that ${\mathrm{O}}^{+}$ is about 5% faster, and ${\mathrm{N}}^{+}$ about 10% faster, than N${\mathrm{O}}^{+}$ in air. $\mathrm{N}_2^{}{}_{}{}^{+}$ is shown to disappear rapidly by charge transfer with ${\mathrm{O}}_2$. Thus, although the classical value of 1.6 is obtained for the mobility of $\mathrm{N}_2^{}{}_{}{}^{+}$ in air, $\mathrm{N}_2^{}{}_{}{}^{+}$ cannot be present long enough at pressures close to atmospheric to account for the classical observations.