Positive-Ion Structure in a TricriticalHe3-He4Mixture

The mobilities of positive and negative ions have been measured in the normal phase of a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture near the tricritical point. Whereas for negative ions nearly the same weak temperature dependence in observed as in pure ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, the mobility of positive ions in the mixture drops sharply by almost a factor of 2 when phase separation is approached. This is attributed to condensation of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ onto the positive-ion surface.