## Abstract

An electron in liquid helium forces open a cavity referred as an electron bubble. These objects have been studied in many past experiments. It has been discovered that under certain conditions other negatively charged objects can be produced but the nature of these “exotic ions” is not understood. We have made a series of experiments to measure the mobility of these objects, and have detected at least 18 ions with different mobility. We also find strong evidence that in addition to these objects there are ions present which have a continuous distribution of mobility. We then describe experiments in which we attempt to produce exotic ions by optically exciting an electron bubble to a higher energy quantum state. To within the sensitivity of the experiment, we have not been able to detect any exotic ions produced as a result of this process. We discuss three possible explanations for the exotic ions, namely impurities, negative helium ions, and fission of the electron wave function. Each of these explanations has difficulties but as far as we can see, of the three, fission is the only plausible explanation of the results which have been obtained.

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## Notes

An earlier experiment by Sommer [3] gave a value of 1.3 eV with an uncertainty which may be as large as 30 %.

See page 62 of ref. [30].

Note that we have also improved the accuracy of the temperature calibration since this thesis was written and so some of the data has been shifted in temperature by a small amount.

Harvard Apparatus, Holliston, Massachusetts 01746.

This is correct if the field penetration through the grids can be neglected.

Lake Shore Cryotronics, Westerville, Ohio 43082.

See the review by Donnelly and Barenghi [42].

For more details, see Wei [43].

In ref. [37] we have used a different form for the pulse shape and obtained similar results for the shape of the background.

For a more detailed calculation of the structure of the normal electron bubble using density functional theory, see ref. [5]. This calculation finds the radius at which the density of the helium reaches one half of its bulk value to be at around 18 Å.

A discussion of this question is provided in the review paper by Shikin [45].

See, for example, McDaniel and Mason [46].

Note that this estimate of the change in radius is based on Eq. 7 and therefore does not take into account any effect of the effective mass.

Suppression of the decay process would also be likely if the size of the bubble in the liquid had to increase for the decay to occur but this is not the case here.

See, for example, Druyvesteyn and Penning [63].

For a discussion of the process in which a barrier is inserted to separate a quantum system into two parts, see [69].

These tips were prepared by the method we have already described in section 2.2.5.

The laser power deposited into the top of the cell will result in a flow of normal fluid down the cell. However, the flow velocity is too small to have a significant effect on the arrival time of the ions.

We thank Wei Guo for this suggestion.

More details about this approach are described in a paper by Z. Xie, W. Wei, Y. Yang, and H.J. Maris, accepted for publication in the Journal of Experimental and Theoretical Physics.

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## Acknowledgments

We thank many colleagues for discussion on the topics presented in this paper. Especial thanks to M. Barranco, A. Ghosh, W. Guo, G.S. Guralnik, D. Jin, A. Vilesov, and G.A. Williams. This work was supported in part by the National Science Foundation under Grant No. DMR 0965728.

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Wei, W., Xie, Z., Cooper, L.N. *et al.* Study of Exotic Ions in Superfluid Helium and the Possible Fission of the Electron Wave Function.
*J Low Temp Phys* **178**, 78–117 (2015). https://doi.org/10.1007/s10909-014-1224-3

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DOI: https://doi.org/10.1007/s10909-014-1224-3

### Keywords

- Electrons
- Ions
- Superfluid