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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.

Great enhancement of molecular nitrogen luminescence in the afterglow of nitrogen-helium gas mixtures was observed at temperatures ≤ 10 K. The effect is explained by the increased efficiency of the recombination of nitrogen atoms and energy transfer from metastable nitrogen molecules and helium atoms to nitrogen molecules in the cold dense helium vapor.Great enhancement of molecular nitrogen luminescence in the afterglow of nitrogen-helium gas mixtures was observed at temperatures ≤ 10 K. The effect is explained by the increased efficiency of the recombination of nitrogen atoms and energy transfer from metastable nitrogen molecules and helium atoms to nitrogen molecules in the cold dense helium vapor.

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A new simple experimental technique has been elaborated to test applicability of impedance spectroscopy for studying processes during destruction of impurity-helium condensates. Combination of methods of optical spectroscopy, impedance spectroscopy and current spectroscopy to study the destruction processes of impurityhelium condensates has been applied for the first time. Experimental data have demonstrated a rather good sensitivity of the technique and proved formation of charged clusters during a destruction stage of impurity-helium condensates. Просту експериментальну методику розроблено та успішно випробувано для використання можливостей спектроскопії імпедансу при дослідженні процесів на стадії руйнування зразків домішково-гелієвих конденсатів. Вперше використано комбінацію методів спектроскопії імпедансу, струмової спектроскопії та оптичної спектроскопії для дослідження руйнування домішково-гелієвих конденсатів. Отримані результати показали високу чутливість нової методики та підтвердили появу зарядів (заряджених нанокластерів) на стадії руйнування домішково-гелієвих конденсатів. Ключові слова: нанокластери, домі Простая экспериментальная методика разработана и успешно опробована для использования возможностей спектроскопии импеданса при исследовании процессов на стадии разрушения образцов примесь-гелиевых конденсатов. Впервые использована комбинация методов спектроскопии импеданса, токовой спектроскопии и оптической спектроскопии для исследования разрушения примесь-гелиевых конденсатов. Полученные результаты показали высокую чувствительность новой методики и подтвердили появление зарядов (заряженных нанокластеров) на стадии разрушения примесьгелиевых конденсатов.

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.

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.

The stabilization and recombination of nitrogen atoms N(4S) in nitrogen-helium and nitrogen–neon-helium condensates obtained by the injection of impurity particles from a gas discharge into bulk superfluid helium are investigated by the EPR method. It is established that the stabilized nitrogen atoms reside inside and on the surface of impurity clusters forming a porous structure in the bulk superfluid helium. The possibility of increasing the specific energy of impurity–helium condensates by increasing their density through mechanical pressing is investigated. For nitrogen-helium condensates an eightfold increase in the specific energy is achieved. The recombination loss of N(4S) upon heating of impurity–helium condensates in the temperature range 1.7–7 K is detected. This permits verification of the mechanism of thermoluminescence of impurity–helium condensates.

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.

The thermoluminescence spectra during the destruction of impurity–helium solid phase samples of different compounds, containing stabilized N and O atoms, have been obtained. For the samples formed by injection of mainly Ar or Kr atoms into bulk superfluid helium (HeII), M-bands of the NO molecule are observed. The green bands appropriate to the E 1 Σ → B 1 Σ transition of the XeO molecule are observed on the destruction of samples containing Xe atoms.

The elemental composition of the impurity–helium solid phase (IHSP) grown by injecting of a gas jet containing Ne, Ar, Kr, and Xe atoms and N2 molecules into superfluid HeII is studied. The measured stoichiometric ratios S=NHe/NIm are much larger than the values predicted by the model of frozen together monolayer helium clusters. The theoretical possibility of freezing together of two-layered clusters is justified in the continual model of the helium subsystem of IHSP which fills the space between rigid impurity centers. Regularities of decomposition of “dry” samples (extracted from liquid helium) are analyzed in the temperature range 1.5–12 K under pressures from 10 to 500 torr. Two stages of sample decomposition are discovered: a slow stage accompanied by cooling and a rapid stage accompanied by heat release. These results suggest the presence of two types of helium in IHSP, viz., weakly bound and strongly bound helium which can be attributed respectively to the second and first coordination spheres of ...

Investigations of metastable N( 2 D) and Ne( 3 P 2 ) atoms isolated in solid helium have been carried out for the first time. The luminescence spectra and kinetics in the impurity helium solid phase (IHSP), with impurity centers N 2 , Ne, Ar, and Kr, show the sensitizing influence of a neighboring heavy particle on the forbidden N( 2 D- 4 S) transition. The extremely long-lived (τ≈10 4 s) hyperbolically decaying afterglow and thermoluminescence of this transition have been observed in the IHSP. Thermoluminescence studies allowed the determination of the energy barrier for pair fusion of neighboring centers. In agreement with calculations the energy barrier of this process which determines the IHSP stability turns out to be E 1 = 40 ± 4 K. The activation energy of the long-lived afterglow stage was found to be E 2 ≈ 7 K, close to the energy of vacancy formation in solid helium. By using laser-induced fluorescence, Ne( 3 P 2 ) atoms have been detected in superfluid helium and in the IHSP for the first time. Within an experimental accuracy of 0.18 A their spectral lines were unshifted and unbroadened with respect to the gas-phase values.

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).

We present optical spectra of impurity-helium condensates during the process of their formation by injection of gas mixtures N2-Rg-He (Rg=Ne, Kr) into bulk superfluid helium after passing through a RF discharge. Atomic lines of He, Ne, Kr, N, O atoms as well as bands (1+ and 2+ systems of N2) are present in the spectral range 320–1100 nm studied in these experiments. We also detected spectra emitted by the samples during their destruction, stimulated by warming through the temperature range 1.5–15 K The luminescence spectra of the N-N2-He and N-N2-Ne-He samples contain only intense N(2D-4S) (α-group) and O(1D-1S) (β-group) lines. For the N-N2-Ne-He sample we observed the transformation of spectra of the α-group. In thermo-stimulated luminescence spectra of the N-N2-Kr-He samples, the intense β-group of O atoms and M-bands of NO molecules were found. Differences in the spectra obtained during destruction of N-N2-Ne-He and N-N2-Kr-He samples may be explained by a different shell structure of the nanoclusters formed during sample preparation.

New experimental results on detection of optical spectra and ion currents during destruction of impurity–helium condensates (IHCs) have been obtained. It is shown that emission during IHC sample destruction is accompanied by current pulses, pressure peaks and temperature changes. The molecular bands of excimer molecules XeO* are assigned to molecules stabilized in films of molecular nitrogen covering the heavier cores of impurity clusters which form impurity–helium condensates.