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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.

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.

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.

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 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.

A small permanent magnet was used to attract impurity-helium solid samples composed of hydrogen, deuterium, and nitrogen radicals. The magnetic field gradient was sufficiently strong to lift each of the impurity-helium solids while submerged in superfluid helium, but only strong enough to lift one of four samples through the liquid surface. This suggests ranges of local atomic radical concentrations that partially agree with previous ESR measurements. The attractive paramagnetic force is strong enough to be useful as a trap for the formation of a pure hydrogen impurity-helium solid, for use in radical concentration measurements and for sorting and moving impurity-helium solids.

Impurity-helium solids are porous gel-like materials held together by Van der Waals forces. They consist of impurity atoms, molecules or clusters of atoms and molecules, each surrounded by very thin layers of solid helium. Impurities studied include neon, krypton, and molecular and atomic nitrogen, hydrogen and deuterium. The pore sizes and cluster sizes in impurity-helium solids are determined by ugtrasound attenuation and X-ray diffraction. The ESR technique is employed to study atomic impurities. The tunnelling exchange chemical reactions D+H2→HD+D and D+HD→D2+H are observed. Large concentrations of atomic hydrogen (∼8⋅1017 per cm3) are produced in these reactions.

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid 4He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ∼8×1020 cm−3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 1019 cm−3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.

We have introduced guest particles into superfluid helium using a directed helium jet containing traces of species under study. The distinguishing peculiarity of the method consists in that the whole system is sealed from the cryostat main helium bath. This allows: (i) on the account of the absence of evaporating helium upflow to realize a complete capture of the impurities from the jet into liquid helium; (ii) to eliminate the dependence of the process conditions on liquid He level position in the main bath as well as on the amount of liquid He condensed inside a cell; and (iii) this method can be used to introduce impurities into liquid 3He. Two modifications of the technique have been designed—one for an optical cryostat and another for a cryostat with narrow 1” tail typical for use in a very high magnetic field. Optical and X-ray diffraction studies have confirmed the possibility of embedding in superfluid helium samples consisting of submicron D2 particles with a rate of 10mmoles per hour. Such samples are necessary for the achievement of strong D2 nuclear spin polarization by the brute force method.

We have performed studies of the formation and evolution of samples of Im-Helium (Im=Ne, D2, N2) solids obtained by allowing a mixed beam of helium and impurity molecules to fall onto the surface of superfluid helium. Clusters consisting of single impurity molecules and/or small aggregates of the molecules surrounded by layers of solid helium agglomerate to form porous solids. These solids have been studied by x-ray diffraction techniques. As the solids were heated above their formation temperature of T=1.5 K, the small aggregates or clusters of impurities tended to form larger clusters, which x-ray diffraction revealed to be nanocrystallites ranging in size from 3 to 6 nm.

1. Section 2.1, second paragraph, fifth line. Sentence is correct as follows: The α′-group corresponds to the electronic transition N(2D → 4S) accompanied by a simultaneous vibrational excitation v = 0 → v = 1 in the neighbouring N2(X1Σ+ g ) molecule. 2. Section 2.2, line 12: a-group and g-line are corrected as α-group and γ -line. 3. Section 3, second paragraph, line 6: W3 −u (v′′ = 4) → A3Σ+ u (v′ = 0–4) is corrected to W3 u(v′′ = 4) → A3Σ+ u (v′ = 0–4) 4. Section 3, second paragraph, line 16: W3 −u is corrected to W3 u 5. Updated Ref. 2 is as follows: V.V. Khmelenko, I.N. Krushinskaya, R.E. Boltnev, I.B. Bykhalo, A.A. Pelmenev, D.M. Lee, Low Temp. Phys. 38, 871 (2012).