Electrons and Exotic Ions in Superfluid Helium-4

We present a review of the properties of negative ions that are produced when electrons are inserted into superfluid helium-4. It is convenient to divide the ions into two groups. The first is the ion usually referred to as the "normal electron bubble," or NEB. The structure of this ion is well understood; it is a small bubble containing a single electron and essentially no helium atoms. The second group is made up of a large number of different ions each with a mobility larger than that of the normal electron bubble. These have been named the "exotic ions," and their structure is unknown.