Effects of impurities on negative ion mobility in O$_{2}$

We have investigated the effects of impurities on the negative ion mobility in O$_{2}$ at atmospheric pressure using a high-pressure ion drift tube with a positive point plate gap that acts as a negative ion detector. We reported a reduced mobility, in particular ``zero field mobility'' of 2.31 cm$^{2}$/V$\cdot$s for O$_{2}^{-}$ in O$_{2}$.\footnote{Y. Okuyama et al.: J. Phys. D: Appl. Phys., 45, 195202 (2012).} This value is in good agreement with values in other reports of 1.95 to 2.42 cm$^{2}$/V$\cdot$s. Although many studies have been carried on the measurement of negative ion mobility over the last 50 years, discrepancies between the values obtained remain and the origin of the discrepancies has not been clarified until now. We found that one of the reasons for the discrepancies originates from impurities in O$_{2}$ that are considered to be released from the surface of the chamber as an absorbed gas (N$_{2}$ or CO$_{2})$ or to already exist in the O$_{2}$. These impurities form negative ions such as CO$_{3}^{-}$, CO$_{4}^{-}$, NO$_{3}^{-}$ and N$_{2}$O$_{2}^{-}$ with the O$_{2}$ in the chamber. The mobilities of these ions are slightly larger than that of O$_{2}^{-}$. Therefore, if small amounts of impurities such as N$_{2}$ and CO$_{2}$ exist in O$_{2}$, an increased negative ion mobility is observed at $E$/$N >$ 2.54$\times $10$^{-1}$ Td. Moreover, the negative ion mobility of O$_{2}^{-}$ was also measured in high-purity O$_{2 }$(99.9999{\%}) and ultrahigh-purity O$_{2}$(99.99995{\%}) while employing a gas filter that can reduce the water content to less than 100 ppt. As a result, the mobility of O$_{2}^{-}$ was increased to 2.39 cm$^{2}$/V$\cdot$s. This value is close to the values reported by Dutton and Howells (J. Phys. B, 1, 1160, '68), Rees (Aust. J. Phys., 18, 41, '65) and Voshall et al. (J. Chem. Phys., 43, 1190, '65).