Structure of negative impurity ions in liquid helium

The experimental mobilities of negative halogen (Cl^?, F^?, and I^?) and metal (Ba^? and Ga^?) impurity ions in superfluid ⁴He are close to each other and much lower than the mobilities of not only He⁺ ions but also electron bubbles. It has been shown that the formation of multiatomic complexes (clusters or bubbles) around ions is responsible for this low mobility. Although the mobilities are similar, the structures of the resulting complexes are qualitatively different in the cases of halogens and metals: solid clusters, which are similar to a well-studied cluster at the He⁺ ion, are formed near halogen ions, which exhibit high electron affinities, whereas metal ions are localized in bubbles, which are similar to electron bubbles. The temperature and pressure dependence of the mobility of these complexes is qualitatively different. Experiments in this area, most likely, performed with a wider variety of negative ions, would enhance the understanding of the structure of charged complexes in liquid helium.