Mobility of alkali earth ions in superfluid helium

Alkali earth ions are utilized as microscopic probes in superfluid helium to study the roton limited mobility in the kinetic regime. A new ion source was used in the gas phase as well as inside the liquid helium (4He). Drift velocities have been measured as a function of the electrical drift field for He+, Ca+, Sr+, Ba+ and for Mg+ ions. Data for the latter ion were not available so far. The results of recent measurements fit well into the picture extrapolated from the other ions.