Ions in Liquid Helium

The mobility of an ion in liquid ${\mathrm{He}}^4$ is known to be appreciably less than the mobility of a ${\mathrm{He}}^3$ atom and varies in a different way with temperature. It is suggested that the essential difference is that electrostriction effects increase the liquid density over a large region surrounding the ion, so that the ion drags around with it about 50 ${\mathrm{He}}^4$ atoms. The density field of the positive ion is calculated in a semiclassical approximation. A possible difference between the density fields for positive and negative ions is briefly discussed. Various experimental ways of checking this picture of an ion are suggested.