Three-Dimensional Shapes of Spinning Helium Nanodroplets

TL;DR

This study uses extreme ultraviolet diffraction imaging to reveal the three-dimensional shapes of spinning superfluid helium nanodroplets, showing classical shape evolution without metastable oblate forms, thus linking morphology and superfluidity.

Contribution

First direct 3D imaging of rotating helium nanodroplets, demonstrating shape evolution from oblate to prolate and challenging previous metastable shape reports.

Findings

Droplets exhibit classical shape transitions during rotation.

Metastable oblate shapes are not observed in superfluid helium droplets.

Shape evolution correlates with angular momentum and superfluid properties.

Abstract

A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion have been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large dataset allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.