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 E/N for 2×10^-16E/N2×10^-14 V cm² and for 1.8×10¹⁵N7×10¹⁵ cm^-3. These measurements lead to values of the zero-field mobilities of 1.6, 3.5, and 2.5 cm²/V sec for N₂^_⁺, NO⁺, and O₂^_⁺, respectively, in air. The high-E/N data yield momentum-transfer cross sections of 110, 21, and 30 ² for N₂^_⁺, NO⁺, and O₂^_⁺, respectively. The rapid disappearance of N⁺ and O⁺ due to ion-molecule reactions prevented good drift-velocity measurements for them, but partial results indicate that O⁺ is about 5% faster, and N⁺ about 10% faster, than NO⁺ in air. N₂^_⁺ is shown to disappear rapidly by charge transfer with O₂. Thus, although the classical value of 1.6 is obtained for the mobility of N₂^_⁺ in air, N₂^_⁺ cannot be present long enough at pressures close to atmospheric to account for the classical observations.