Recent Progress in Studies of Nanostructured Impurity–Helium Solids

Impurity–helium (Im–He) solids are porous materials formed inside superfluid ⁴He by nanoclusters of impurities injected from the gas phase. The results of studies of these materials have relevance to soft condensed matter physics, matrix isolation of free radicals and low temperature chemistry. Recent studies by a variety of experimental techniques, including CW and pulse ESR, X-ray diffraction, ultrasound and Raman spectroscopy allow a better characterization of the properties of Im–He solids. The structure of Im–He solids, the trapping sites of stabilized atoms and the possible energy content of the samples are analyzed on the basis of experimental data. The kinetics of exchange tunneling reactions of hydrogen isotopes in nanoclusters and the changes of environment of the atoms during the course of these reactions are reviewed. Analysis of the ESR data shows that very large fraction of the stabilized atoms in Im–He solids reside on the surfaces of impurity nanoclusters. The future directions for studying Im–He solids are described. Among the most attractive are the studies of Im–He solids with high concentrations of stabilized atoms at ultralow (10–20 mK) temperature for the observation of new collective quantum phenomena, the studies of practical application of Im–He solids as a medium in neutron moderator for efficient production of ultracold (∼1 mK) neutrons, and the possibilities of obtaining high concentration of atomic nitrogen embedded in N₂ clusters for energy storage.     

Thermal Decomposition Mechanism of N2-Impurity Helium Solids

The mechanism for the thermal decomposition of N₂ containing impurity helium solids (IHS) in superfluid ⁴He is reported. We show that the solid undergoes rapid thermal annealing leading to mobilization of the nitrogen atoms. Subsequently, the nitrogen atoms form molecular N₂ excimers, which are mobile in the solid and are responsible for various emission bands observed over the thermal decomposition of IHS. The present interpretation of the spectroscopic results indicates that IHS contains only ground state atomic species.PACS numbers: 33.20 Kf, 33.70 Jg     

Generation of intrinsic excitations in superfluid 4He by using gas jet approach

No Heading A radio frequency discharged helium gas jet was used to generate intrinsic excitations in bulk superfluid ⁴He. The present experimental results and our previous estimates show that metastable triplet state He atoms and excimers cannot enter directly the liquid but rather concentrate on the liquid surface. High concentration of metastable species promotes reactions, which then lead to formation of He ions. Upon electron - ion recombination, population of highly excited atomic and diatomic excimer electronic states occurs. Effect of molecular hydrogen on quenching of the helium emissions is demonstrated. Excitations are efficiently transferred from He to H₂.PACS numbers: 33.20 Kf, 33.70 Jg.     

Structural Studies of Nanocrystalline Nitrogen–Helium Solids by Raman Spectroscopy

Structural and thermal properties of nanocrystalline nitrogen–helium solids are studied by Raman spectroscopy at temperature range 1.8–41 K. The N₂ vibrational line possesses spectral structure very similar to what is observed in bulk solid nitrogen, indicating ordered structure inside the nanocrystallites. The spectral observations show that the structure of the solid is dependent on the N₂ content in the gas mixture used for sample preparation. Evidence for disordered nitrogen on the surfaces and interfaces of nanocrystallites can be extracted from the Raman spectra of the most diluted samples. Removing superfluid helium from the sample and annealing at 21 K did not affect the structure of the solid, whereas higher annealing temperatures yielded strong increase of density as evidenced by the up to ∼10-fold increase of Raman signal. The α-solid—β -solid transition, evidenced by a ∼0.8 cm⁻¹ shift in the peak position, was seen in the collapsed nitrogen samples approximately at the same temperature as in a solid nitrogen film.