Investigation of the Fast Negative Ion in Superfluid Helium-4
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Investigation of the Fast Negative Ion in Superfluid Helium-4 S. Sirisky1 · Y. Xing1
· G. M. Seidel1 · H. J. Maris1
Received: 11 July 2019 / Accepted: 11 December 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract In 1970, Doake and Gribbon reported the discovery of a negative ion in superfluid helium that has a mobility about six times higher than the mobility of the normal electron bubble. We report on measurements of the variation of the mobility of this “fast ion” with applied pressure. Keywords Liquid helium · Fast ion · Mobility
1 Introduction There are now at least 19 different negative ions that have been detected in superfluid helium-4. However, for only one of these, the so-called normal electron bubble, or NEB, is the structure known. As a result of many experiments, it is clear that the NEB is a bubble of radius about 19 Å containing an electron and no helium atoms. This structure was first proposed by Careri [1,2] following on from a proposal by Ferrell [3] to explain the anomalously long lifetime of positronium in liquid helium. The 18 other ions have a higher mobility than the NEB and have been seen in several experiments [4–13]. Doake and Gribbon were the first to report the existence of one of these ions [4]. The ion they detected has the highest mobility of all of the ions, about six times the mobility of the NEB, and is referred to as the “fast ion”. There is no generally accepted theory of these ions, see discussion in Ref. [13], and for some more recent work, see Refs. [14,15]. Clearly more experimental results are needed to clarify the situation. So far, only two properties of the ions have been studied: (i) The ion mobility has been measured in the temperature range from 1 K to about 1.2 K. In most of the experiments, the ions have been produced by creating a plasma in the saturated vapor above the surface of the liquid. The density of the
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Y. Xing [email protected] Department of Physics, Brown University, Providence, RI 02912, USA
123
Journal of Low Temperature Physics
vapor increases rapidly as the temperature goes up, and above 1.2 K, it is hard to maintain a stable plasma. The energy dissipated in the plasma makes it hard to perform experiments below 1 K. (ii) Measurements have been taken of the critical velocity at which moving ions nucleate quantized vortices and become trapped [8,9]. It was found that this critical velocity was larger for the exotic ions of higher mobility. For the fast ion, vortex nucleation was not observed even when the ion velocity approached the Landau critical velocity. From these reports, it is not clear whether the fast ion is simply the exotic ion that has the highest mobility or whether it is an ion that has a different structure from the other exotic ions. In favor of the latter view, we note that in the experiment of Doake and Gribbon [4] a radioactive α source instead of a plasma was used to generate ions, and they reported the mobility of only the fast ion. In another experiment, we have found that when we
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