Cluster Aggregation in Bosonic and Fermionic Helium Nanodroplets*

Bosonic superfluid helium-4 droplets obtained in molecular beams have sizable angular momentum, which manifests itself physically in the form of quantized vortices. In our previous works, vortices have been visualized via ultrafast X-ray coherent diffractive imaging using xenon as a contrast agent. Xenon atoms are attracted to the vortex cores leading to the formation of filament – shaped clusters [1]. Here, we study the formation of clusters in non-superfluid fermionic helium-3 droplets, which also rotate but are devoid of any quantum vortices. It was found that in distinction to helium-4, xenon atoms inside helium-3 form loose aggregates arranged into rings on the periphery of the droplets. This is because in normal fluid helium-3 droplets, dopants are fixed in the rotating frame. Thus, heavy xenon atoms are pushed to the droplets’ periphery by centrifugal forces where they recombine into ring-shaped structures.