Coagulation of Metals in Superfluid and Normal Liquid Helium

Abstract

The thermal emission study in this work has shown that coagulation of metals in liquid helium is accompanied by enormous local overheating of several thousand degrees. Direct experiments demonstrated, for the first time, that condensation of metals in superfluid helium occurs via the specific mechanism which is substantially faster than that in normal liquid helium. It has been stated that coagulation of metals in superfluid helium indeed occurs in two stages, a "hot" one of nanoparticles coalescence with the formation of molten nanospheres and the subsequent stage of their sticking together into nanowires. It turned out that if a laser ablation of metal targets immersed in superfluid helium was used for introducing a metal into liquid, the formation of nanowires occurs at distances of only about 1 mm from the laser focus. This leads to the presence of a considerable number of spherical inclusions in nanowires grown in such a way.

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…When exploring the mass spectra of HNDs doped with methyl iodide, doubly charged ions of the form HenI 2+ were discovered with relatively high abundance [367]. The energy required to form I 2+ from CH3I is 32.05 eV for the reaction CH3I + e - I 2+ +CH3 + 3e -, (10) or 31.97 eV if CH3is formed and two electrons are emitted [368]. Even when considering the solvation energy of the He atoms surrounding I 2+ , a single He + ion cannot form HenI 2+ ions, because more than 30 eV of energy is required.…”

“…This rapid phase separation has severely limited the study of molecules in bulk liquid helium. Work that has been done has been largely limited to small metal clusters, delivered into the liquid by laser ablation techniques (see for example [6][7][8][9][10]).…”

The nanowires growth by laser ablation of metals inside rotating superfluid helium

“…At the second stage, surface energy released upon merging of two cold clusters begins to be insufficient for complete melting of the coagulation product, so the metal clusters of critical size can only stick together in the core of vortex, thereby forming a nanowire. Under typical experimental conditions, the first stage, which subsequently determines the diameter of the nanowire produced at the second stage, lasts no more than 25 µs [24]. Because the ballistic motion of particles in superfluid helium is possible only at velocities not exceeding the critical velocity (about 50 m s −1 ) [25] the formation of the wire occurs already at a distance of about one millimeter from the surface of the filled by plasma gas bubble existed around laser beam spot.…”

“…In addition, it is positive that in such experimental setting the ablation is located outside the liquid helium, and thus laser pulses could not create in the liquid neither shock waves [27], nor the powerful local overheating [24]. However, it becomes necessary now to ensure that the distance between the ablation point and He II level is remaining constant throughout the experiment.…”