Positive impurity ions in He II

We have developed a technique for introducing a variety of impurity atomic ions into liquid helium and measuring their mobility. We find that, in contrast with predictions of the "snowball" model of Atkins, the ion mobility depends on the atomic number of the core ion in a way that cannot be explained simply in terms of the mass of the ion. The relative mobility of ⁴⁰Ca and ⁴⁸Ca ions has been measured and is consistent with existing theories.