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

TL;DR

This study investigates dynamic nuclear polarization and spin relaxation of hydrogen and deuterium atoms in solid mixtures, revealing mechanisms like the Overhauser and Cross effect, with temperature-independent electron relaxation and temperature-dependent nuclear relaxation.

Contribution

It provides new insights into DNP mechanisms and relaxation processes in solid hydrogen isotope mixtures under high magnetic fields and low temperatures.

Findings

Nuclear relaxation time decreases with temperature, consistent with the Orbach mechanism.

Efficient DNP achieved via the Overhauser and Cross effect mechanisms.

Clusters of H atoms with exchange interactions enable DNP through partially allowed ESR transitions.

Abstract

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.