Highly dispersed, unsupported Pt, Pd, Rh, and bimetallic R h P t and P t P d clusters were produced by laser ablation using a KrF excimer laser operating at power densities up to 1.9 GW/cm2. By the evaporation of two metals simultaneously, bimetallic clusters with variable stoichiometric composition were obtained, depending on buffer gas, buffer gas pressure, and the deposited laser energy. The ablation was carried out in a glass cell under a continuous gas flow or in a UHV system with a static gas atmosphere. The pressure ranged from 5 to 100 Torr. Emission spectra of the plasma plume were taken for detection of ions and highly excited atoms in order to study the temporal evolution of the ablation process. The expansion of the plasma plume was imaged with a CCD camera at different times after the laser pulse. In the first few nanoseconds, the expansion velocity reached values up to 3.5 x lo6 c d s . Characterization of the metal clusters by XPS showed that samples produced in the glass reactor are highly contaminated with carbon up to 60 atom %. However, by using a UHV system, the carbon content was reduced to 14 atom %. Alloy formation in bimetallic Rh/Pt particles with a l-to-1 atomic ratio of rhodium to platinum was verified by EXAFS spectroscopy. The coordination environment around platinum consists of both platinum and rhodium neighbors, 56 and 44 atom %, respectively, while rhodium appears to be coordinated predominantly to other rhodium atoms. These results are consistent with a bimetallic Rh/Pt cluster model whereby the cluster core is composed of rhodium atoms and the outer surface is formed by platinum atoms. Particle size and size distribution were analyzed by XRD, TEM, and BET isotherms. The particle diameters ranged from 2 to 20 nm, showing a linear dependence on buffer gas pressure and laser energy.

Fragmentation and the subsequent heat transfer from solvated gold nanoparticles with sizes 50, 100, 150, and 250 nm to water are studied by time-resolved shadowgraph photography. The measured fragmentation thresholds indicate that the particles heat up to temperatures between the boiling point and critical temperature of gold. At high laser energy densities, complete fragmentation of the particles is observed, which is evidenced by the sudden breakdown of the thermally insulating Leidenfrost layer initially surrounding the laser-heated particles. The consequent heat transfer results in the production of water vapor, which expands through pressure−volume work to form a micrometer-sized bubble. The spatially resolved shadowgraph images of these bubbles show the exact locations of the parent nanoparticles in the solution. Furthermore, a simple thermodynamic model can be used to relate the observed maximum bubble diameters to the original nanoparticle sizes. Although the presented method for online analysis of plasmonic metal nanoparticles in the liquid phase is destructive, it can be implemented in practice to employ a single laser pulse with a small beam size to achieve only minimal perturbation of the sample.

The dynamics following laser ablation of a metal target immersed in superfluid $^4$He is studied by time-resolved shadowgraph photography. The delayed ejection of hot micrometer-sized particles from the target surface into the liquid was indirectly observed by monitoring the formation and growth of gaseous bubbles around the particles. The experimentally determined particle average velocity distribution appears similar as previously measured in vacuum but exhibits a sharp cutoff at the speed of sound of the liquid. The propagation of the subsonic particles terminates in slightly elongated non-spherical gas bubbles residing near the target whereas faster particles reveal an unusual hydrodynamic response of the liquid. Based on the previously established semi-empirical model developed for macroscopic objects, the ejected transonic particles exhibit supercavitating flow to reduce their hydrodynamic drag. Supersonic particles appear to follow a completely different propagation mechanism as they leave discrete and semi-continuous bubble trails in the liquid. The relatively low number density of the observed non-spherical gas bubbles indicates that only large micron-sized particles are visualized in the experiments. Although the unique properties of superfluid helium allow a detailed characterization of these processes, the developed technique can be used to study the hydrodynamic response of any liquid to fast propagating objects on the micrometer-scale.

Properties of shock waves and solitons in superfluid 4 He were studied by time-resolved shadowgraph experiments and theoretical density functional theory calculations. Pressure estimates for shock waves and the bright soliton limit (0.2 MPa) were provided and compared with the semiclassical Rankine-Hugoniot theory. Overall, the shock wave amplitude-velocity relationship was observed to be linear at least up to 175 kg/m 3. At high amplitudes, the shock waves decay into sound waves in the wake as well as a bright soliton train in the front. This suggests that the experimental shadowgraph data in Phys. Rev. Lett. 120, 035302 (2018) corresponds to such a train structure rather than an individual bright soliton. With reference to theoretical calculations, a new approach based on accelerating wall embedded in the liquid is proposed for generating single solitons in superfluid helium. This process is also predicted to produce dark solitons in superfluid helium, which have not yet been observed experimentally. At low soliton amplitudes, collision with an exponentially repulsive wall results in nearly lossless reflection, which is accompanied by soliton inversion from dark to bright or bright to dark.

Typeset by REVT E X 1 arXiv:1807.08464v1 [cond-mat.other] Abstract Formation of vortex rings around moving spherical objects in superfluid 4 He at 0 K is modeled by time-dependent density functional theory. The simulations provide detailed information of the microscopic events that lead to vortex ring emission through characteristic observables such as liquid current circulation, drag force, and hydrodynamic mass. A series of simulations were performed to determine velocity thresholds for the onset of dissipation as a function of the sphere radius up to 1.8 nm and at external pressures of zero and 1 bar. The threshold was observed to decrease with the sphere radius and increase with pressure thus showing that the onset of dissipation does not involve roton emission events (Landau critical velocity), but rather vortex emission (Feynman critical velocity), which is also confirmed by the observed periodic response of the hydrodynamic observables as well as visualization of the liquid current circulation. An empirical model, which considers the ratio between the boundary layer kinetic and vortex ring formation energies, is presented for extrapolating the current results to larger length scales. The calculated critical velocity value at zero pressure for a sphere that mimics an electron bubble is in good agreement with the previous experimental observations at low temperatures. The stability of the system against symmetry breaking was linked to its ability to excite quantized Kelvin waves around the vortex rings during the vortex shedding process. At high vortex ring emission rates, the downstream dynamics showed complex vortex ring fission and reconnection events that appear similar to those seen in previous Gross-Pitaevskii theory-based calculations, and which mark the onset of turbulent behavior.PACS numbers: