On the formation mechanism of impurity–helium solids: evidence for extensive clustering

Abstract

Optical emission studies on a discharged nitrogen-helium gas jet injected into superfluid helium near 1.5 K are described. The analysis of atomic (a-group) and molecular Vegard-Kaplan transitions clearly indicates that the emitting species are embedded in the nitrogen clusters. The formation of the clusters is most efficient in the crater formed on the liquid surface. The model calculations based on the classical bubble model and density functional theory suggest that under the experimental conditions only clusters consisting of more than 1000 molecules have a kinetic energy sufficient for the stable cavity formation inside liquid helium. The results obtained suggest that the formation of impurity-helium solids is a consequence of extensive clustering in the gas jet.

Atomic and molecular defects in solid 4He

“…Luminescence spectra from metastable nitrogen and oxygen atoms, as well as from molecular nitrogen from samples produced by the analogous technique were also reported by a group at Jyväskylä University [229,244]. The authors compared the spectra of N and N 2 observed in nitrogen-helium solids to those in a pure solid nitrogen matrix and find them to be very similar, thus supporting the conclusion that the emitting particles in the nitrogenhelium solid are in fact surrounded by solid nitrogen and not by helium, i.e., that they are embedded in nitrogen clusters.…”

“…However, later experimental studies of the impurityhelium solid structure using X-ray diffraction, ultrasound propagation and electron-spin resonance (ESR) techniques, have shown that the typical impurity-helium solid consists of rather large impurity clusters. The cluster formation mechanism was discussed in [229,131], with the conclusion that the clusters are already formed in the gas jet before hitting the liquid He surface. A solid shell of He is then formed around each cluster due to the van der Waals attraction and prevents their further coalescence into larger particles.…”

Observation of the fcc-to-hcp Transition in Ensembles of Argon Nanoclusters

“…This method, known as the impurity-helium gas injection technique, 27 works for rare gases, as well as for many other chemical species (the "impurity"). Separated clusters grow via coalescence of the impurity atoms and their solidification and thermalization in a helium vapor 28 . They are stabilized in liquid helium and remain separated or in minimum contact, arguably protected by formation of strongly-attached "coats" of helium atoms.…”

Instability of Small Deuterium Clusters in Superfluid Helium near the λ Point

“…Separated clusters grow by means of coalescence of impurity atoms or molecules and their solidification and thermalization in a helium vapor. Being injected into superfluid helium they are stabilized due to low temperatures, the fast heat removal via superfluid helium and also owing coverage by one or two layers of helium atoms, which separate clusters from each other [9]. The suggested paper is devoted to our study of intriguing processes observed in the vicinity of the λ point in ensembles of deuterium clusters immersed in superfluid helium applying the small-angle quasielastic neutron scattering technique accompanied by the precise analysis of the data.…”

“…They may be prepared in supersonic jets [1,2], porous media [3], gas aggregation experiments [4] but production of large amounts of isolated clusters is a very challenging task. The impurity-helium gas injection technique (IHGIT) known since 1970s [5] is an unique method for resolving this problem [6][7][8][9][10][11][12]. Separated clusters grow by means of coalescence of impurity atoms or molecules and their solidification and thermalization in a helium vapor.…”