Study of Electrons in Superfluid Helium

Brown University, Providence RI

Investigators

Abstract

Non-technical Abstract When an electron is injected into helium, it forms a very small bubble (diameter ~38 Å) from which helium atoms are excluded. This "electron bubble" forms a unique quantum system which has been extensively studied and is fairly well understood. However, in addition to the normal electron bubbles other negatively charged objects are produced whose origin remains a mystery. These objects, which have been called exotic ions, have higher mobility and appear to be smaller than the normal electron bubbles and, despite much work, the nature of these objects remains a mystery. One possible explanation, which remains somewhat controversial, is that these exotic ions are bubbles in the liquid which contain only a part of the wave function of a single electron. This project is aimed at exploring the nature and structure of these exotic ions using ultrasonic, optical and transport measurements. If it can be established that these are indeed bubbles with only a fraction of the electron wave function it will result in a radical change in how physicists view quantum theory. This research project is ideally suited for the training of undergraduate and graduate students who will learn a broad range of practical skills including electronics, vacuum techniques, low temperature physics, optics and computer programming. Technical Abstract In this research our team investigates exotic negatively-charged ions which have been discovered in superfluid helium at temperatures around 1 K. The goal of the research is to discover the inner structure of these exotic ions. In previous experiments our group has made mobility measurements which show that there are at least 18 different ions, each with a different characteristic size. Remarkably, there are also ions which have a continuous size distribution. Despite much work by different research groups the structure of these ions remains unknown. There is strong experimental evidence that they are not impurities, and it appears impossible to relate them to atomic or molecular helium ions previously detected in helium gas. One remaining possibility is that the ions are electron bubbles each containing only a fraction of the wave function of a single electron. While this is a radical idea, it does provide a way to understand the existence of a large number of different ions and makes it possible for there to be ions with a continuous distribution of size. The goal of the current project is to learn more about the properties of the ions and in this way to test the idea that bubbles containing a fraction of the wave function can exist as stable entities. In a first project a negative pressure is applied to the liquid and the magnitude of the pressure required to cause a selected exotic ion to explode is measured. The magnitude of the negative pressure is then compared with theoretical predictions. In a second project the optical absorption by the ions is measured. This determines the photon energies which can induce optical excitation of the ions and gives a second test of the theory. The research project uses a wide range of techniques including ultrasonics, optics, low temperature, and computer simulation and provides excellent training for undergraduate and graduate students.