Trapped-ion motion in helium II

We present the results of a series of experiments on the mobility of negative ions trapped on quantized vortex lines in superfluid helium. Trapped-ion mobilities were measured as a function of temperature, pressure, and He³ concentration. Some of the experimental data have been previously reported.^1,2 The temperature and pressure dependence of trapped-ion mobilities is qualitatively different than that of free ions. The He³ data strongly suggest that the negative ion bubble does not become deformed on the vortex line. We present a model for a vortex line having a central "core" of normal fluid extending over a distance of several angstroms surrounded by a "tail," a region of excess roton density whose momenta are predominantly aligned opposite to the direction of circulation of the superfluid. This model is used to calculate the drag on a negative ion trapped on a vortex line. The model appears to account for the experimental results satisfactorily.