Periodic Discontinuities in the Drift Velocity of Ions in Liquid Helium II

The drift velocity of positive and negative ions in liquid helium around 1°K has been studied by the time-of-flight method as a function of the applied field. Sharp and periodic discontinuities have been detected at values which are integer multiples of $〈v_c〉=5.2\mathrm{}\pm 0.2$ m/sec for positive ions, and $〈v_c〉=2.4\mathrm{}\pm 0.1$ m/sec for negative ones. Metastable mobility levels induced by bath disturbances have also been detected. Analysis of these results, together with the ones obtained by the heat-flush method, shows this process to be due to single ions which suffer additional interaction with the roton gas when their drift velocity relative to the background fluid reaches the observed critical velocities. A tentative model is proposed which explains these results by assuming vortex rings of different quantum numbers of circulation to be closely bound to the ion in motion, which interact with the roton gas via their velocity field.