No. Kiry1160 Beamline(s): X20A Nanostructured materials are currently of immense interest in both fundamental and applied science. Recently, we have reported 1 the structural and thermal properties of novel mesoporous inert-gas solids. The materials were prepared by injecting a jet of helium containing dilute amounts of inert atoms and molecules (Ne, Kr, N 2) into superfluid helium. A special x-ray cryostat equipped with a gas preparation system allowed us to study these materials with x-ray diffraction. In the initial experiments, we found that complex mesoporous substances with densities of the order of 10 20 impurity atoms per cm 3 with the characteristic size of the constituent building block of 6 nm form in the superfluid helium. The Kr and Ne solids were stable outside of liquid He up to temperatures above 10 K. In this work, we continue to investigate nanoporous inert-gas solid samples with x-ray diffraction techniques. We report successful preparation of samples made of deuterium (D 2), and also deuterium-nitrogen mixed samples. By varying preparation conditions, we were able to achieve synthesis of nitrogen samples with the characteristic size of the building block less than 3 nm. Our experiments indicate that by varying the experimental conditions, it is possible to control the size (and, possibly, morphology) of the nanostructured inert solids. We also demonstrate that these solids can be made of a large variety of chemical elements. In addition to the standard powder diffraction measurements, we have also performed small-angle scattering studies of the porous inert-gas samples. Our measurements indicate that the samples to not possess a well-defined fractal structure, and that the nanoscale morphology of the samples substantially varies from sample to sample. These materials may find various applications as a new type of porous medium for fundamental physics, as well as in cluster physics, matrix isolation spectroscopy, and catalysis of low-temperature chemical reactions. Better control of the sample properties achieved in our measurements is an essential first step towards these possible applications.

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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 ͑ESR͒ techniques. X-ray-diffraction measurements show that these solids are highly porous gel-like structures consisting of D 2 clusters with the characteristic cluster size of 90Ϯ30 Å. The densities of D 2 molecules in the samples are 7ϫ10 20 -3ϫ10 21 cm Ϫ3 . 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 1ϫ10 18 cm Ϫ3 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 2ϫ10 19 cm Ϫ3 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.

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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.

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