Observation of a New Type of Negative Ion in Superfluid Helium

In recent work, we have developed a new technique for the study of the properties of electron bubbles (negative ions) in liquid helium. We use ultrasound to measure the critical negative pressure P_c at which an electron bubble becomes unstable and explodes. The value of P_c is affected, for example, by the quantum state of the electron and is reduced if the bubble is attached to a quantized vortex. In the present experiments, we have discovered a new type of object that appears to be larger than the usual electron bubble. We will consider possible explanations of these observations.