We present the first magnetic resonance study of atomic hydrogen embedded in solid H2 films for temperatures 150-900 mK. We found that at T approximately 150 mK average concentrations of H atoms of order 10(18 cm(-3) are very stable against recombination during two weeks of observations. The distribution of the population of the two lowest hyperfine states is found to be non-Boltzmann, with a very large occupation of the ground state. We consider the possibility of formation in solid H2 of regions with high local concentrations of H atoms, where collective quantum phenomena might occur.

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.

We present an experimental study of H atoms embedded in thin films of solid H 2 at temperatures below 1 K. H 2 films up to 50 nm thick were first grown as a result of slow recombination of atomic hydrogen gas on the sample cell walls. If the recombination occurred in three-body atomic collisions in the gas phase, small concentrations of atoms could be captured inside the films during the film deposition. As a second method of generating atomic populations inside the H 2 films, we used a direct dissociation by a low power rf discharge in the sample cell. With this latter method, we achieved record high atomic concentrations exceeding 2 ϫ 10 19 cm −3 . The samples were characterized by means of magnetic resonance: electron spin resonance ͑ESR͒ and electron-nuclear double resonance ͑ENDOR͒ in a magnetic field of 4.6 T. We observed density-dependent broadening and shifts of the ESR lines due to the dipolar interactions, and resolved these effects for like and unlike atoms. Relaxation of the relative hyperfine populations was measured as a function of temperature for H in H 2 films grown on different substrates. For H 2 films on Mylar substrates, the relative equilibrium populations of the two lowest hyperfine states of H were found to deviate substantially from the prediction of Boltzmann statistics. We also found two narrow lines in the ENDOR spectra of H in H 2 films shifted to the red from the position for free atoms. This indicates two possible substitutional positions of the atoms in H 2 matrices, both characterized by very homogeneous crystalline fields.