We have produced ultracold (0.37 K) superfluid helium droplets containing between 10<sup>5 </sup>and 10<sup>10 </sup>He atoms and subjected them to electron beams of defined energy and current. Mass per charge distributions were determined by electrostatic energy analyzers for both positively and negatively charged droplets. Utilizing two ionization sources in series, each followed by a spherical sector field electrostatic analyzer, it was possible to unambiguously determine the charge state as well as the mass of charged He droplets. Contrary to what has often been believed, we find that most of the charges remain in these weakly bound droplets instead of being efficiently expelled. Charge states higher than 65 for positively charge droplets and 5 for negatively charge droplets could be assigned, and the charge states of the droplets could be further manipulated (both increased and decreased) with the second ion source. Perfect fractional numbers of the mass per charge ratios of product ions with respect to the charged precursor droplets (see panel b the figure) indicate that fragmentation exclusively happens via very asymmetric Coulomb explosions and evaporation of He atoms, with the mass of the parent droplets decreasing only very slightly. These low-mass ions have been investigated by mass spectrometer systems in past experiments. Coulomb repulsion between charge centers of the same polarity in highly-charged He droplets will lead to minimum energy configurations in the form of Coulomb crystals with the charges near the surface of the droplets. Dopants will be more readily polarized and attracted by the charged centers than the surrounding He atoms, which thereby act as seeds for cluster growth.

Highly charged droplets of superfluid helium

2019 Fall National Meeting

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