Three-Dimensional Shapes of Spinning Helium Nanodroplets

Abstract

A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion has 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 data set 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.

Quantum coherent diffractive imaging

“…The comparison of the respective (spectrally integrated) scattering profiles in figure 3 documents the agreement in the linear response limit and validates our method. The scattering images show the typical ring structure for spherical droplets [11].…”

“…While CDI with hard x-rays for maximal spatial resolution is likely to remain the domain of XFELs, lab-based sources have become sufficiently brilliant to enable CDI in the XUV domain. Despite lower nominal spatial resolution, XUV-CDI enables the measurement of wide-angle scattering and thus provides three-dimensional structural information [7], which is particularly valuable for the characterization of non-reproducible targets [11].…”

Deep neural networks for classifying complex features in diffraction images

“…Helium nanodroplets [1] were imaged using extreme ultraviolet (XUV) photon energies between 19 eV to 35 eV using the experimental setup of the LDM beamline [42,43] at the Free Electron Laser FERMI [44]. Scattering images were recorded with a multi-channel-plate (MCP) detector combined with a phosphor screen which was placed 65 mm downstream from the interaction region; this defines the maximum scattering angle of 30 • .…”

“…Out of 2 × 10 5 laser shots, about 38 000 images were obtained. The images were corrected for straylight background and the flat detector (see also Langbehn et al [1])…”

“…We can then interpret this as a reliable indicator for a failed generalization of the network. The physics behind these patterns are quite complicated as well, but for a rigorous interpretation and analysis of these patterns, please see Langbehn et al [1].…”