Mobility of Negative Ions in O2 at Low E/N

Measurements of negative-ion mobilities in pure O2 were made as a function of gas pressure (2–700 Torr), gas temperature (77.4° to 373°K), and electric field. Values of E/N, electric field over gas density, ranged from 6×10−19 to 6×10−17 V·cm2 so that the ions were in thermal equilibrium with the gas. The ion mobility times gas density μN is independent of density for 195°K and above. Values of μN for this ion, O2−, are 5.7×1019, 6.5×1019, 6.8×1019 V−1·sec−1·cm−1 for temperatures of 195°, 300°, and 373°K, respectively. The density dependence of the measured mobilities at 77.4° and 87.6°K is consistent with an equilibrium between the concentrations of O2− and O4− ions. The low-density data yield values of μN for the O2− ion of 4.7×1019 and 4.9×1019 V−1·sec−1·cm−1 at 77.4° and 87.6°K. The μN value of O4− ions is 2.6±0.5×1019 V−1·sec−1·cm−1 at both temperatures. Correlation with models for the internal motion of O4− leads to an estimated dissociation energy of 0.06±0.02 eV.