Measurement of negative ion mobility varying with a little amount of H$_{2}$O in O$_{2}$

A study of transport properties such as a mobility of charged particles is importance in understanding discharge plasmas. These fundamental data have been collected in some databases. The authors have been investigated the effects of impurities such as H$_{2}$O, CO$_{2}$, N$_{2}$ on the negative ion mobility in O$_{2}$ at high pressures including atmospheric pressure [1, 2]. Especially, the effect of a trace amount of H$_{2}$O in O$_{2}$ could not to be avoided for the mobility measurement due to formation of cluster ions O$_{2}^-$ $\cdot$ (H$_{2}$O)$_{\mathrm{n}}$ (n $=$ 1, 2, 3, ...). In this study, the mobility of negative ions was measured in O$_{2}$ varying with the H$_{2}$O concentration from 100 to 17000 ppb. The H$_{2}$O concentration was monitored with a trace moisture analyzer whose operation was based on a photoabsorption method. As the results, a constant mobility of 2.39 cm$^{2}$/V$\cdot$s was observed in ultrahigh-purity O$_{2}$ (99.99995{\%}, purified with a gas purifier), in which the H$_{2}$O concentration was monitored to be between 15 and 100 ppb. This value was good agreement with the mobility of O$_{4}^-$ in previous report [1]. Then, a small amount of H$_{2}$O from 2000 to 17000 ppb was added to the ultrahigh-purity O$_{2}$. Two kinds of mobilities 2.31 and 2.21 cm$^{2}$/V$\cdot$s were observed in H$_{2}$O concentration ranges of 2000 - 4600 and 4600 - 17000 ppb, respectively. Former one is good agreement with the mobility observed in high-purity O$_{2}$ (99.9999{\%}) [2]. These mobilities are considered to be those of O$_{2}^-$ $\cdot$ (H$_{2}$O)$_{\mathrm{n}}$ (n $=$ 1, 2). [1] Y. Okuyama et al, 66th Annual Gaseous Electronic Conference, 57, 8, MR1.00091 (2013). [2] Y. Okuyama et al, J. Phys. D: Appl. Phys., 45, 195202 (2012).