氢同位素固体混合物中H和D原子的动态核极化和弛豫
Dynamic nuclear polarization and relaxation of H and D atoms in solid mixtures of hydrogen isotopes
摘要：
本文报道了H $_{2}$、D $_{2}$和HD混合物固体分子基质中氢和氘原子的动态核极化和电子及核自旋弛豫的研究。电子和核自旋弛豫时间（$T_{1e}$和$T_{1N}$）在4.6 T的磁场中在0.15-2.5 $\,$ K的温度范围内测量，该条件确保电子自旋的高极化。我们发现，$T_{1e}$在此温度范围内几乎与温度无关，而$T_{1N}$降低了2个数量级。这种强烈的温度依赖性是典型的通过电子自旋弛豫的核奥巴赫机制。我们发现，在固体D-$_{2}$和D-$_{2}:$ HD中，H原子的核自旋可以被Overhauser效应有效地极化.泵浦H原子的禁戒跃迁也导致DNP，效率强烈依赖于D原子的浓度。这种行为表明了DNP和核弛豫的交叉效应机制，这在我们的实验条件下得到了很好的解决。当抽运位于ESR谱中心的中间D线时，也观察到H原子的有效DNP。这一现象可以用具有强交换作用的H原子团簇或对来解释。这些簇在ESR谱的中心部分地允许跃迁，并且DNP可以通过分辨的交叉效应来创建。
英文摘要：
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.
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