Dynamic nuclear polarization and relaxation of H and D atoms in solid mixtures of hydrogen isotopes

Original · EN

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 H₂, D₂, and HD mixtures. The electron and nuclear spin relaxation times (T₁ₑ and T₁ₙ) were measured within the temperature range 0.15-2.5K in a magnetic field of 4.6 T, conditions which ensure a high polarization of electron spins. We found that T₁ₑ is nearly temperature independent in this temperature range, while T₁ₙ 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 D₂ and D₂: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.