Mobility of Xe+ in Xe-Ne and Xe-He mixtures

We report on the measurement of the mobility of Xe+ in Xe-Ne and Xe-He mixtures over the density-reduced electric field strength E/N, from 6.5 to 300 Td (1 Td = 10-17 V cm2), and Xe concentrations in the range 2-50%. The measurements are carried out with a pulsed Townsend apparatus (PT). Even though the PT apparatus lacks mass analysis, the measurements are thoroughly justified in terms of mass identification by ensuring that the majority ion species is Xe+ over carefully selected ranges of E/N where this ion is overwhelmingly predominant over either Ne+ or He+, the presence of which is limited to a maximum of 1% in this study. Ample evidence is given that the contribution of additional Xe+ to that provided by electron impact ionization via the charge transfer reaction Ne+(He+) + Xe+ → Ne(He) + Xe+ exerts a negligible effect on the evaluation of the drift velocity. Likewise, the finite bandwidth of the amplifier is shown not to influence the drift velocity (mobility) measurements at all. For all the mixtures under study the equilibration times are substantially smaller than the associated ion transit times so that full assurance is given to having the ion swarms in equilibrium with the neutral gas mixtures. No previously measured mobilities of Xe+ in these mixtures are found; however, there is good agreement between our measurements and those previously calculated.