Experimental Investigation of Exotic Negative Ions in Superfluid Helium

We have constructed a new apparatus designed to study the exotic negative ions in superfluid helium-4 previously observed. Our apparatus is similar to that used by Ihas and Sanders, and by Eden and McClintock. The ions are generated from an electrical discharge in the vapor above the surface of the liquid and the mobility is measured by the time-of-flight method. We have detected eleven exotic ions with distinct mobility, and have measured their mobility as a function of temperature. In addition to the peaks in the time-of-flight signal due to the different exotic ions, there is a smoothly varying and continuous background signal. The variation of the background with field and temperature appears to be inconsistent with any explanation invoking the decay of one exotic ion into another, and supports the idea it arises from negative ions with a continuous distribution of mobility. This is a striking result because it would indicate that these ions have a continuous size distribution.