Generation of Large Vortex-Free Superfluid Helium Nanodroplets.

Superfluid helium nanodroplets are an ideal environment for the formation of metastable, self-organized dopant nanostructures. However, the presence of vortices often hinders their formation. Here, we demonstrate the generation of vortex-free helium nanodroplets and explore the size range in which they can be produced.

Three-dimensional femtosecond snapshots of isolated faceted nanostructures.

The structure and dynamics of isolated nanosamples in free flight can be directly visualized via single-shot coherent diffractive imaging using the intense and short pulses of x-ray free-electron lasers. Wide-angle scattering images encode three-dimensional (3D) morphological information of the samples, but its retrieval remains a challenge. Up to now, effective 3D morphology reconstructions from single shots were only achieved via fitting with highly constrained models, requiring a priori knowledge about possible geometries.

Anisotropic Surface Broadening and Core Depletion during the Evolution of a Strong-Field Induced Nanoplasma.

Strong-field ionization of nanoscale clusters provides excellent opportunities to study the complex correlated electronic and nuclear dynamics of near-solid density plasmas. Yet, monitoring ultrafast, nanoscopic dynamics in real-time is challenging, which often complicates a direct comparison between theory and experiment. Here, near-infrared laser-induced plasma dynamics in ∼600  nm diameter helium droplets are studied by femtosecond time-resolved x-ray coherent diffractive imaging.

Sizes of pure and doped helium droplets from single shot x-ray imaging.

Advancements in x-ray free-electron lasers on producing ultrashort, ultrabright, and coherent x-ray pulses enable single-shot imaging of fragile nanostructures, such as superfluid helium droplets. This imaging technique gives unique access to the sizes and shapes of individual droplets. In the past, such droplet characteristics have only been indirectly inferred by ensemble averaging techniques.

Aggregation of solutes in bosonic versus fermionic quantum fluids.

Quantum fluid droplets made of helium-3 (He) or helium-4 (He) isotopes have long been considered as ideal cryogenic nanolabs, enabling unique ultracold chemistry and spectroscopy applications. The droplets were believed to provide a homogeneous environment in which dopant atoms and molecules could move and react almost as in free space but at temperatures close to absolute zero. Here, we report ultrafast x-ray diffraction experiments on xenon-doped He and He nanodroplets, demonstrating that the unavoidable rotational excitation of isolated droplets leads to highly anisotropic and inhomogeneous interactions between the host matrix and enclosed dopants.

The 1-Megapixel pnCCD detector for the Small Quantum Systems Instrument at the European XFEL: system and operation aspects.

The X-ray free-electron lasers that became available during the last decade, like the European XFEL (EuXFEL), place high demands on their instrumentation. Especially at low photon energies below 1 keV, detectors with high sensitivity, and consequently low noise and high quantum efficiency, are required to enable facility users to fully exploit the scientific potential of the photon source. A 1-Megapixel pnCCD detector with a 1024 × 1024 pixel format has been installed and commissioned for imaging applications at the Nano-Sized Quantum System (NQS) station of the Small Quantum System (SQS) instrument at EuXFEL.

Disintegration of diminutive liquid helium jets in vacuum.

The phenomenon of liquid jets disintegrating into droplets has attracted the attention of researchers for more than 200 years. An overwhelming fraction of these studies considered classical viscous liquid jets issuing into ambient atmospheric gases, such as air. Here, we present an optical shadowgraphy study of the disintegration of a cryogenic liquid helium jet produced with a 5 µm diameter nozzle into vacuum.

Angular Momentum in Rotating Superfluid Droplets.

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid ^{4}He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host.

Helium superfluidity. Shapes and vorticities of superfluid helium nanodroplets.

Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, exploring the dynamic properties of individual droplets is experimentally challenging. In this work, we used single-shot femtosecond x-ray coherent diffractive imaging to investigate the rotation of single, isolated superfluid helium-4 droplets containing ~10(8) to 10(11) atoms.