Potential of an ionic impurity in a large 4He cluster

This paper presents an analysis of the motion of an impurity ion in a nanometre scale 4He cluster. Due to induction forces, ions are strongly localized near the centre of the cluster, with a root mean squared thermal displacement of only a few A. The trapping potential is found to be nearly harmonic, with a frequency of 2.3(1.0) GHz for a positive (negative) ion in a He cluster of radius 5nm. The anharmonicity is small and positive (energy increases slightly faster than linear with quantum number). It is suggested that by using frequency sweep microwave radiation, it should be possible to drive the ion centre-of-mass motion up to high quantum numbers, allowing the study of the critical velocity as a function of cluster size.