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Characterisation of a pulsed superfluid liquid helium droplet beam source

James Harries, Arisa Iguchi, Susumu Kuma

Published Jun 24, 2024• 0 citations
OpenAlex

Abstract

We have developed a new pulsed superfluid liquid helium droplet beam source for use as a target source for imaging and quantum optics experiment at a synchrotron or x-ray free-electron laser beamline. The characteristics of the source have been investigated using the titration method to estimate droplet sizes for different source temperatures and pressures.

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We have developed a new pulsed superfluid liquid helium droplet beam source for use as a target source for imaging and quantum optics experiment at a synchrotron or x-ray free-electron laser beamline. The characteristics of the source have been investigated using the titration method to estimate droplet sizes for different source temperatures and pressures.
Free superfluid helium droplets constitute a versatile medium for a diverse range of experiments in physics and chemistry that extend from studies of the fundamental laws of superfluid motion to the synthesis of novel nanomaterials. In particular, the emergence of quantum vortices in rotating helium droplets is one of the most dramatic hallmarks of superfluidity and gives detailed access to the wave function describing the quantum liquid. This review provides an introduction to quantum vorticity in helium droplets, followed by a historical account of experiments on vortex visualization in bulk superfluid helium and a more detailed discussion of recent advances in the study of the rotational motion of isolated, nano- to micrometer-scale superfluid helium droplets. Ultrafast X-ray and extreme ultraviolet scattering techniques enabled by X-ray free-electron lasers and high-order harmonic generation in particular have facilitated the in situ detection of droplet shapes and the imaging of vortex structures inside individual, isolated droplets. New applications of helium droplets ranging from studies of quantum phase separations to mechanisms of low-temperature aggregation are discussed.
Liquid 4He^{4}\mathrm{H}\mathrm{e}4He at pressures P0=0.5−−30  bars{P}_{0}=0.5--30\text{ }\text{ }\mathrm{\text{bars}}P0​=0.5−−30  bars and temperatures T0=1.5−−4.2  K{T}_{0}=1.5--4.2\text{ }\text{ }\mathrm{K}T0​=1.5−−4.2  K is discharged into vacuum through two different $2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ nozzles. The velocities of the beam of particles obey the Bernoulli equation down to 15  m/sec15\text{ }\text{ }\mathrm{m}\mathrm{/}\mathrm{s}\mathrm{e}\mathrm{c}15  m/sec. With decreasing T0{T}_{0}T0​ and increasing P0{P}_{0}P0​ the velocity and angular distributions become exceedingly narrow with $\ensuremath{\Delta}v/v\ensuremath{\lesssim}1%$ and $\ensuremath{\Delta}\ensuremath{\vartheta}\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{m}\mathrm{r}\mathrm{a}\mathrm{d}$. Optical observations indicate that the beam consists of micron-sized droplets ($N\ensuremath{\gtrsim}{10}^{9}\text{ }\text{ }\mathrm{\text{atoms}}$). This new droplet source provides opportunities for novel experimental studies of superfluid behavior.
We present two facile time-of-flight (TOF) methods of detecting superfluid helium droplets and droplets with neutral dopants. Without an electron gun and with only a heated filament and pulsed electrodes, the electron impact ionization TOF mass spectrometer can resolve ionized helium clusters such as He2(+) and He4(+), which are signatures of superfluid helium droplets. Without ionizing any helium atoms, multiphoton non-resonant laser ionization of CCl4 doped in superfluid helium droplets at 266 nm generates complex cluster ions of dopant fragments with helium atoms, including (He)(n)C(+), (He)(n)Cl(+), and (He)(n)CCl(+). Using both methods, we have characterized our cryogenic pulsed valve—the Even-Lavie valve. We have observed a primary pulse with larger helium droplets traveling at a slower speed and a rebound pulse with smaller droplets at a faster speed. In addition, the pickup efficiency of dopant is higher for the primary pulse when the nozzle temperature is higher than 13 K, and the total time duration of the doped droplet pulse is only on the order of 20 μs. These results stress the importance of fast and easy characterization of the droplet beam for sensitive measurements such as electron diffraction of doped droplets.
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. Here, we report on the size distributions of both pure and doped droplets collected from single-shot x-ray imaging and produced from the free-jet expansion of helium through a 5 μm diameter nozzle at 20 bars and nozzle temperatures ranging from 4.2 to 9 K. This work extends the measurement of large helium nanodroplets containing 109–1011 atoms, which are shown to follow an exponential size distribution. Additionally, we demonstrate that the size distributions of the doped droplets follow those of the pure droplets at the same stagnation condition but with smaller average sizes.
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. The formation of quantum vortex lattices inside the droplets is confirmed by observing characteristic Bragg patterns from xenon clusters trapped in the vortex cores. The vortex densities are up to five orders of magnitude larger than those observed in bulk liquid helium. The droplets exhibit large centrifugal deformations but retain axially symmetric shapes at angular velocities well beyond the stability range of viscous classical droplets.
Abstract In this introductory chapter, we begin by informing the reader about the fascinating history of superfluidity in bulk liquid helium. This is followed by relating attempts in using liquid helium as a low temperature matrix for spectroscopy. After a brief review of the thermodynamic properties of helium in Sect. 1.2, the different types of free jet expansions used in experiments to produce clusters and nanodroplets of different sizes are described in Sect. 1.3. First it is shown how they depend on the nature and location in the phase diagram of the isentropes which determine the course of the expansion. Depending on the four regimes of isentropes, different number sizes and distributions are expected. Next in Sect. 1.4, the results of theoretical and, where available, experimental results on the total energies, excited states, radial density distributions, and temperatures of clusters and droplets are discussed. Finally, in Sect. 1.5 the theoretical and experimental evidence for the superfluidity of nanodroplets is briefly reviewed. For more information on the production and characteristics of nanodroplets, the reader is referred to the chapters in this book and to the reviews in Appendix.