In addition to the normal electron bubbles there are other objects called exotic ions. The physical nature of these ions is not understood. They were first seen in experiments performed by Doake and Gribbon , Ihas and Sanders , and McClintock and coworkers . So far they have been studied only by mobility measurements. Twelve of these objects have been detected, each with a different mobility which is higher mobility than the mobility of the normal electron bubbles. The fastest ion (called the "fast ion") has a mobility about seven times larger than the normal electron bubbles. The higher mobility implies that these bubbles are smaller than normal electron bubbles.
These ions have been detected in experiments in which electrons entered the liquid from a plasma formed by an electrical discharge in the vapor above the liquid surface; the fast ion has been seen also when an alpha source  was used for generation.
Here is a schematic drawing of a new apparatus we have built to study the exotic ions.
High voltages are applied to the electrodes in the upper part of the cell and an electrical discharge is produced in the helium vapor. Electrons enter the liquid from the vapor. The dc voltages on the gating grids G1 and G2 are set to prevent negatively-charged ions passing through. By applying a negative pulse to G1 the gate is opened for a few hundred microseconds and a pulse of ions enters the drift region below G2. The ions arriving at the collector plate C give a voltage which is amplified and then averaged.
Here is a plot of the detected signal as a function of time. Each peak rises from a different exotic ion. 
The apparatus has been constructed so that measurements can be made under stable conditions for extended periods of time. This is an important improvement compared to the earlier experiments and has enabled us to make very careful measurements of the mobility of the different ions as a function of temperature and to investigate how the strength of the signals due to the different exotic ions vary with conditions in the discharge.
The results we have obtained indicate that we are seeing the same objects as discovered in the earlier experiments. This makes it very unlikely that the exotic ions are associated with impurities. We are continuing these experiments to improve the signal to noise and see if there are other ions, giving weak signals, which perhaps have not yet been detected.
- ^C.S.M. Doake and P.W.F. Gribbon, Phys. Lett. 30A, 251 (1969).
- ^G.G. Ihas and T.M. Sanders Phys. Rev. Lett. 27 383 (1971); G.G. Ihas, Ph.D. thesis, University of Michigan, 1971; G.G. Ihas and T.M. Sanders, in Proceedings of the 13th International Conference on Low Temperature Physics, editors K.D. Timmerhaus, W.J. O'Sullivan and E.F. Hammel, (Plenum, New York, 1972), Vol. 1, p. 477.
- ^V.L. Eden and P.V.E. McClintock, Phys. Lett. 102A 197 (1984); C.D.H. Williams, P.C. Hendry and P.V.E. McClintock, Jap. J. Appl. Phys. 26-3 105 (1987).
- ^W. Wei, Z. Xie, G.M. Seidel, and H.J. Maris, "Experimental Investigation of Exotic Negative Ions in Superfluid Helium", paper to be presented at the Quantum Fluids and Solids conference, 2012.