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We report on a study of Dynamic Nuclear Polarization and electron and nuclear spin relaxation of atomic hydrogen and deuterium in solid molecular matrices of H$_{2}$, D$_{2}$, and HD mixtures. The electron and nuclear spin relaxation times ($T_{1e}$ and $T_{1N}$) were measured within the temperature range 0.15-2.5$\,$K in a magnetic field of 4.6 T, conditions which ensure a high polarization of electron spins. We found that $T_{1e}$ is nearly temperature independent in this temperature range, while $T_{1N}$ decreased by 2 orders of magnitude. Such strong temperature dependence is typical for the nuclear Orbach mechanism of relaxation via the electron spins. We found that the nuclear spins of H atoms in solid D$_{2}$ and D$_{2}:$HD can be efficiently polarized by the Overhauser effect. Pumping the forbidden transitions of H atoms also leads to DNP, with the efficiency strongly dependent on the concentration of D atoms. This behaviour indicates the Cross effect mechanism of the DNP and nuclear relaxation, which turns out to be well resolved in the conditions of our experiments. Efficient DNP of H atoms was also observed when pumping the middle D line located in center of the ESR spectrum. This phenomenon can be explained in terms of clusters or pairs of H atoms with strong exchange interaction. These clusters have partially allowed transitions in the center of the ESR spectrum and DNP may be created via the resolved Cross effect.

We report on the first ESR study of atomic hydrogen and tritium stabilized in a solid T$_{2}$ and T$_{2}$:H$_{2}$ matrices down to 70$\,$mK. The concentrations of T atoms in pure T$_{2}$ approached $2\times10^{20}$cm$^{-3}$ and record-high concentrations of H atoms $\sim1\times10^{20}$cm$^{-3}$ were reached in T$_{2}$:H$_{2}$ solid mixtures where a fraction of T atoms became converted into H due to the isotopic exchange reaction T+H$_2\rightarrow$TH+H. The maximum concentrations of unpaired T and H atoms was limited by their recombination which becomes enforced by efficient atomic diffusion due to a presence of a large number of vacancies and phonons generated in the matrices by $\beta$-particles. Recombination also appeared in an explosive manner both being stimulated and spontaneously in thick films where sample cooling was insufficient. We suggest that the main mechanism for H and T migration is physical diffusion related to tunneling or hopping to vacant sites in contrast to isotopic chemical reactions which govern diffusion of H and D atoms created in H$_{2}$ and D$_{2}$ matrices by other methods.

We report on a study of the exchange tunneling reaction D+HD$\rightarrow$D$_{2}$+H in a pure solid HD matrix and in a D$_{2}$ matrix with 0.23$\%$ HD admixture at temperatures between 130$\,$mK and 1.5$\,$K. We found that the exchange reaction rates, $k_{exHD}\sim3\times10^{-27}$cm$^{3}$s$^{-1}$ in the pure HD matrix, and $k_{exD_{2}}=9(4)\times10^{-28}$cm$^{-3}$ in the D$_{2}$ matrix are nearly independent of temperature within this range. This confirms quantum tunnelling nature of these reactions, and their ability to proceed at temperatures down to absolute zero. Based on these observations we concluded that exchange tunneling reaction H+H$_{2}\rightarrow$H$_{2}$+H should also proceed in a H$_{2}$ matrix at the lowest temperatures. On contrary, the recombination of H atoms in solid H$_{2}$ and D atoms in solid D$_{2}$ is substantially suppressed at the lowest temperatures as a result of increasing of violation for resonance tunneling of atoms when they approach each other.

We report on experimental observation of BEC-like behaviour of quantized electron spin waves (magnons) in a dense gas of spin polarized atomic hydrogen. The magnons are trapped and controlled with inhomogeneous magnetic fields, and described by a Schr\"odinger-like wave equation, in analogy to the BEC experiments with neutral atoms. We have observed the appearance of a sharp feature in the ESR spectrum displaced from the normal spin wave spectrum. We believe that this observation corresponds to a sudden growth of the ground state population of the magnons and emergence of their spontaneous coherence for hydrogen gas densities exceeding a critical value, dependent on the trapping potential. We interpret the results as a BEC of non-equilibrium magnons which were formed by applying the rf power., Comment: 5 pages, 3 figures Supplemental material: 3 pages, 3 figures. Accepted for publication in Physical Review Letters

Impurity-helium solids are formed when a mixture of impurity and helium gases enters a volume of superfluid helium. Typical choices of impurity gas are hydrogen deuteride, deuterium, nitrogen, neon and argon, or a mixture of these. These solids consist of individual impurity atoms and molecules as well as clusters of impurity atoms and molecules covered with layers of solidified helium. The clusters have an imperfect crystalline structure and diameters ranging up to 90 angstroms, depending somewhat on the choice of impurity. Immediately following formation the clusters aggregate into loosely connected porous solids that are submerged in and completely permeated by the liquid helium. Im-He solids are extremely effective at stabilizing high concentrations of free radicals, which can be introduced by applying a high power RF dis- charge to the impurity gas mixture just before it strikes the super fluid helium. Average concentrations of 10(exp 19) nitrogen atoms/cc and 5 x 10(exp 18) deuterium atoms/cc can be achieved this way. It shows a typical sample formed from a mixture of atomic and molecular hydrogen and deuterium. It shows typical sample formed from atomic and molecular nitrogen. Much of the stability of Im-He solids is attributed to their very large surface area to volume ratio and their permeation by super fluid helium. Heat resulting from a chance meeting and recombination of free radicals is quickly dissipated by the super fluid helium instead of thermally promoting the diffusion of other nearby free radicals.