Modelling the mobility of positive ion clusters in normal liquid helium over large pressure ranges

Positively charged helium clusters, also called ‘snowballs’, have been investigated within normal liquid helium. Thermodynamic state equations for ionic helium clusters in liquid helium have been developed, allowing us to discern the ‘hydrodynamic’ radius for a wide range of hydrostatic pressures and temperatures. The mobilities derived from the cluster sizes using stokes law match experimental data with unsurpassed accuracy. For low pressures the compressibility of the cluster ions was found to be distinctly larger than the compressibility of solid helium suggesting that in this pressure range clusters are fully or partially liquid.