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We reviewed systems formed in liquid and solid helium-4 at high concentrations (1016 cm−3 and higher) of impurity particles. A new explanation for the reversible coagulation of fine hydrogen particles into big flakes observed upon the transition of liquid helium into the superfluid phase is proposed. The importance of nanoclusters presence in helium crystals doped with impurity particle for the metastability of icebergs is discussed.

In this work we studied optical spectra of thermoluminescence accompanying the destruction of collections of molecular nitrogen nanoclusters containing stabilized nitrogen, oxygen, and deuterium atoms. These collections of nanoclusters were formed by the injection of products of a radio-frequency discharge in deuterium-nitrogen-helium gas mixtures into bulk superfluid 4He. In the range from 200 to 1650 nm, the bands at 336 nm, 473 nm along with a new band at 1170 nm as well as the known bands of atomic nitrogen and oxygen, and bands of molecular nitrogen, oxygen, and NO were observed. These three bands were assigned to the emission of the ND radicals formed due to recombinations of nitrogen atoms in excited metastable states and deuterium atoms in the ground state during the destruction of ensembles of molecular nitrogen nanoclusters.

Impurity-helium condensates (IHCs) created by injection of nitrogen atoms and molecules as well as rare gas (RG) atoms (Ne, Ar, and Kr) into superfluid $$^4$$ He have been studied via electron spin resonance (ESR) techniques. We investigated the influence of addition of rare gas atoms (Ne, Ar, and Kr) into the condensing $$\hbox {N}_2$$ –He gas mixture on the efficiency of stabilization as well as on the local and average concentrations of N atoms attainable in IHCs. Addition of Ar and Kr atoms into the condensing $$\hbox {N}_2$$ –He gas mixture substantially increased the stabilization efficiency of N atoms in nanoclusters forming IHCs. Measurements of the ground-state spectroscopic parameters of nitrogen atoms show that the nanoclusters have a shell structure. Most of the N atoms reside on solid molecular layers of $$\hbox {N}_2$$ . These layers form on the surfaces of RG (Ar or Kr) nanoclusters.

A multimodal dusty plasma formed in a positive column of the direct current glow discharge at superfluid helium temperatures has been studied for the first time. Formation of a liquid-like dusty plasma structure occurred after injection of polydisperse cerium oxide particles in the glow discharge. The coupling parameter ~10 determined for the dusty plasma structure corresponds very well to its liquid-like type. The cloud of nanoparticles and non-linear waves within the cloud were observed at T 6е, levitating in the gas discharge at the temperature ~2 K and pressure 4 Pa have been observed for the first time. Analysis of the experimental conditions and the filament composition allows us to conclude that the filaments and nanoclusters were formed due to ion sputtering of dielectric material during the experiments.

In this study argon nanoclusters (800 to $$\sim $$ 6500 atoms) formed in supersonic gas jets are compared to the nanoclusters stabilized in superfluid helium. High-energy electron and X-ray diffraction methods are utilized. Both techniques allow investigation of isolated clusters. It is shown that the theoretical prediction of the so-called multiply twinned particles with fivefold symmetry, such as icosahedra (ico) and decahedra (dec) is valid in the investigated cluster size interval. Around the point of the expected ico-to-dec size-dependent transformation at a cluster size of $$\sim $$ 2000 atoms, hexagonal ico and the statistical distribution of structures with a tendency for dec to replace ico are observed. Kinetic reasons, as well as temperature-related effects, could be responsible for the latter observations.

We studied optical and electron spin resonance spectra during destruction of porous structures formed by nitrogen–rare gas (RG) nanoclusters in bulk superfluid helium containing high concentrations of stabilized nitrogen atoms. Samples were created by injecting products of a radio frequency discharge of nitrogen–rare gas–helium gas mixtures into bulk superfluid helium. These samples have a high energy density allowing the study of energy release in chemical processes inside of nanocluster aggregates. The rare gases used in the studies were neon, argon, and krypton. We also studied the effects of changing the relative concentrations between nitrogen and rare gas on thermoluminescence spectra during destruction of the samples. At the beginning of the destructions, $$\alpha $$ -group of nitrogen atoms, Vegard–Kaplan bands of $$\hbox {N}_2$$ molecules, and $$\beta $$ -group of O atoms were observed. The final destruction of the samples were characterized by a series bright flashes. Spectra obtained during these flashes contain M- and $$\beta $$ -bands of NO molecules, the intensities of which depend on the concentration of molecular nitrogen in the gas mixture as well as the type of rare gas present in the gas mixture.

We studied the dynamics of thermoluminescence during destruction of porous structures formed by nanoclusters of nitrogen molecules containing high concentrations of stabilized nitrogen atoms. The porous structures were formed in bulk superfluid helium by injection of the products of discharges in nitrogen–helium gas mixtures through the liquid helium surface. Fast recombination of nitrogen atoms during warming-up led to explosive destruction of the porous structures accompanied by bright flashes. Intense emissions from the $$\alpha $$ -group of nitrogen atoms, the $$\beta $$ -group of oxygen atoms and the Vegard–Kaplan bands of N $$_2$$ molecules were observed at the beginning of destruction. At the end of destruction the M- and $$\beta $$ -bands of NO molecules as well as bands of O $$_2$$ molecules were also observed. Observation of the emissions from NO molecules at the end of destruction was explained by processes of accumulation of NO molecules in the system due to the large van der Waals interaction of NO molecules. For the first time, we observed the emission of the O $$_2$$ molecules at the end of destruction of the porous nitrogen structures as a result of the (NO) $$_2$$ dimer formation in solid nitrogen and subsequent processes leading to the appearance of excited O $$_2$$ molecules.

We studied the luminescence of molecular nitrogen nanoclusters containing stabilized nitrogen, oxygen, hydrogen, and deuterium atoms. Optical spectra were observed during the destruction of these ensembles of nanoclusters accompanied by a rapid release of chemical energy stored in the samples. Several interesting features were observed including a broad band near λ ≈ 360 nm, which was identified as emission corresponding to 2Ag→1Ag transition of N4(D2h) polymeric nitrogen. Also the sharp lines at λ ∼ 336 and 473 nm were observed, and their assignments to ND radicals are discussed.

We have studied thermo-stimulated luminenscence and electron emission of nitrogen films and nanoclusters containing free radicals of atomic nitrogen. Thermo-stimulated electron emission from N2 nanoclusters was observed for the first time. Thermo-stimulated luminescence spectra obtained during the destruction of a N2–He sample are similar to those detected from N2 films pre-irradiated by an electron beam. This similarity reveals common mechanisms of energy transfer and relaxation. The correlation of luminescence intensity and electron current in both systems points to the important role of ionic species in relaxation cascades. Sublimation of solid helium shells isolating nitrogen nanoclusters is a trigger for the initiation of thermo-stimulated luminescence and electron emission in these nitrogen–helium condensates.

The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warmup of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium.

Impurity–helium condensates (IHCs) containing nitrogen and krypton atoms immersed in superfluid 4He have been studied via CW electron spin resonance (ESR). The IHCs are gel-like aggregates of nanoclusters composed of impurity species. It was found that the addition of krypton atoms to the nitrogen–helium gas mixture used for preparation of IHCs increases the efficiency of stabilization of nitrogen atoms. We have achieved high average (5 · 1019 cm−3) and local (2 · 1021 cm−3) concentrations of nitrogen atoms in krypton–nitrogen–helium condensates. The analysis of ESR lines shows that in krypton-nitrogen nanoclusters three different sites exist for stabilization of nitrogen atoms. Nitrogen atoms are stabilized in the krypton core of nanoclusters, in the nitrogen molecular layer that covers the Kr core, and on the surface of the nanoclusters. High concentrations of nitrogen atoms achieved in IHCs provide an important step in the search for magnetic ordering effects at low temperatures.

Injection of an impurity-helium gas jet passed through a radiofrequency discharge into a volume of superfluid helium leads to the growth of nanoclusters of impurity species which form impurity-helium condensates (IHCs). IHCs are porous materials with very low impurity density (∼1020 cm−3). High average concentrations of stabilized free radicals can be achieved on the large total surface (∼100 m2/cm3) of impurity nanoclusters. Warming of the IHCs leads to the destruction of the samples and formation of excited atoms and molecules as a consequence of the recombination of stabilized free radicals. We studied the influence of the nitrogen content in neon-helium and krypton-helium gas mixtures on the thermoluminescence spectra accompanying the destruction of the IHC samples, which were formed by using these gas mixtures. The energy release channels in the IHC samples were revealed from analysis of the thermoluminescence spectra.

Impurity–helium condensates formed by krypton nanoclusters containing atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance (ESR) technique. Analysis of the ESR spectra has shown that most of the H and D atoms reside on the surfaces of Kr nanoclusters. Very large average concentrations have been obtained for H atoms (1.2⋅10⋅19cm−3) and D atoms (3.3⋅19cm−3) in these experiments. Line narrowing has been observed for the highest concentration of D atoms stabilized in the Kr–He sample. Exchange tunneling reactions have been studied in Kr–He sample containing H and D atoms.

We observed the appearance of a broad feature at 360 nm in spectra obtained during the destruction of ensembles of molecular nitrogen nanoclusters containing stabilized nitrogen, oxygen, hydrogen and deuterium atoms. We attribute this broad feature with a maximum at 360 nm to the N4 polynitrogen molecules which are the product of the neutralization reaction of ions with electrons in solid N2.

Impurity–helium condensates containing krypton atoms and also atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance (ESR) technique. Analysis of the ESR spectra shows that most of the H and D atoms reside in molecular layers (H2 or D2) formed on the surfaces of Kr nanoclusters. The thickness of the molecular films was found to determine the rates of recombination of the atoms into molecules, with atoms in the thinner films recombining much more slowly. Very large average concentrations were obtained for H atoms (1019 cm−3) and D atoms (3⋅1019 cm−3) in these experiments.

We have studied superfluid helium capture in a sample of silica aerogel of 98.2% porosity in the temperature range from 1.22 K up to 1.89 K. The high retention of He in the aerogel sample corresponds to a similar phenomenon in impurity-helium condensates, in which very high values of the ratio of helium atoms to impurity atoms (up to 60) have been seen. We have observed that removing the aerogel sample from superfluid helium in a cylindrical glass beaker caused a decrease of the helium level corresponding to the geometrical volume of the sample (≈1 cm3). This observation has allowed us to conclude that superfluid helium is completely captured by the porous sample. Superfluid helium filling aerogel and impurity-helium samples (porous structures) serves as a dispersive medium of gel-like samples which interacts strongly with impurity nanoclusters forming the dispersing system.

We employed en electron spin resonance (ESR) technique for investigating the long term behavior of hydrogen and deuterium atoms in the HD-D2 impurity helium solids that were created by sending a gas mixture [H2]:[D2]:[He]= 1:4:100 through a radio-frequency electrical discharge into a volume of superfluid 4He at T = 1.5 K. H and D atoms were stabilized inside nanoclusters of impurity molecules. The exchange tunneling reactions D + H2 → HD + H and D + HD→D2 + H proceeded to eliminate D atoms and increase the concentration of H atoms in the HD-D2 impurity-helium solids. Local concentrations of H atoms inside the molecular nanoclusters of order 1020 cm−3 were achieved. The high concentration of H atoms was stable during ∼ 40 hours storage of the sample at T = 1.35 K. These solids are possible candidates for collective quantum phenomena of atomic hydrogen if the Bose-Einstein degeneracy regime can be attained.

We present the first observations of excimer XeO* molecules in molecular nitrogen films surrounding xenon cores of nanoclusters. Multishell nanoclusters form upon the fast cooling of a helium jet containing small admixtures of nitrogen and xenon by cold helium vapor (T = 1.5 K). Such nanoclusters injected into superfluid helium aggregate into porous impurity-helium condensates. Passage of helium gas with admixtures through a radio frequency discharge allows the storage of high densities of radicals stabilized in impurity-helium condensates. Intense recombination of the radicals occurs during destruction of such condensates and generates excited species observable because of optical emission. Rich spectra of xenon-oxygen complexes have been detected upon destruction of xenon-nitrogen-helium condensates. A xenon environment quenches metastable N((2)D) atoms but has a much weaker effect on the luminescence of N((2)P) atoms. Electron spin resonance spectra of N((4)S) atoms trapped in xenon-nitrogen-helium condensates have been studied. High local concentrations of nitrogen atoms (up to 10(21) cm(-3)) stabilized in xenon-nitrogen nanoclusters have been revealed.

Macroscopic ensembles of weakly interacting argon nanoclusters are studied using x-ray diffraction in low vacuum. As the clusters grow by fusion with increasing temperature, their structure transforms from essentially face-centered cubic (fcc) to hexagonal close packed as the cluster size approaches $\ensuremath{\sim}{10}^{5}$ atoms. The transformation involves intermediate orthorhombic phases. These data confirm extant theoretical predictions. They also indicate that growth kinetics and spatial constraints might play an important role in the formation of the fcc structure of bulk rare-gas solids, which still remains puzzling.

Impurity-helium condensates (IHCs) containing krypton and hydrogen atoms immersed in superfluid $^{4}\text{H}\text{e}$ have been studied via cw electron-spin-resonance (ESR) techniques. The IHCs are gel-like aggregates of nanoclusters composed of krypton and hydrogen atoms. We have found that very high average $(\ensuremath{\sim}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3})$ and local $(\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3})$ concentrations of hydrogen atoms can be stabilized in these samples. The analysis of ESR line positions and shapes shows that most of the H atom population resides in solid ${\text{H}}_{2}$ layers on the krypton nanocluster surfaces. High concentrations of atomic hydrogen achieved in IHCs provide an important step in the search for collective magnetic and quantum statistical phenomena at lower temperatures.

Macroscopic samples (volume approximately cm(3), atomic density approximately 10(19) -10(20) cm(-3)) of noble-gas nanoclusters (size approximately 5-6 nm) were produced in superfluid helium by an impurity-helium gas injection technique. X-ray diffraction measurements show that the samples consist of weakly interacting nanoclusters with fivefold symmetry, such as icosahedra and decahedra. These results open new opportunities for fundamental research of nanoclusters of noble gases and other materials in well-controlled environments using experimental techniques requiring bulk samples.

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid 4 He have been studied via x-ray-diffraction and electron-spin-resonance (ESR) techniques. X-ray-diffraction measurements show thatthese solids are highly porous gel-like structures consisting of D 2 clusters with the characteristic cluster size of 90′ 30 A. The densities of D 2 molecules in the samples are 7 × 10 2 0 -3 × 10 2 1 cm - 3 . Each of the D 2 clusters are either partially or totally surrounded by thin layers of adsorbed helium which may play an important role in preventing the coalescence of the clusters into larger crystallites of solid D 2 . Using ESR, we find that average concentrations of D atoms of order I × 10 1 8 cm - 3 can be achieved in our samples. Measurements of the ground-state spectroscopic parameters and relaxation times of atomic deuterium show that the D atoms reside in the D 2 clusters. The combined x-ray and ESR data show that local concentrations of D atoms as large as 2 × 10 1 9 cm - 3 are obtained in our experiments. The highly porous deuterium nanostructures studied in this work are promising for the production of high concentrations of ultracold neutrons and for significant nuclear polarization of D 2 molecules by the "brute force method" at low temperatures.

Impurity-Helium condensates (IHCs) containing krypton and hydrogen atoms have been studied in superfluid helium-4 via CW ESR techniques. The IHCs studied in this work are gel-like aggregates of nanoclusters composed of krypton and hydrogen atoms. We have found that such samples contain very high average concentrations of hydrogen atoms (~1018cm-3) as obtained by integration of the microwave absorption signal. Local concentrations (~1019cm-3) of H atoms were calculated from the ESR line width. Detailed studies of the ESR line shapes lead to the conclusion that a large fraction of the H atoms lies on the krypton cluster surfaces.