Steiner Jakob M, Hautle Patrick, Wenckebach W Tom
Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
J Magn Reson. 2021 Dec;333:107099. doi: 10.1016/j.jmr.2021.107099. Epub 2021 Oct 29.
Under typical conditions for dynamic nuclear polarization (DNP)-temperature about 1 K or below and magnetic field about 3 T or higher-the polarization agent causes nuclear dipolar order to relax up to four orders of magnitude faster than nuclear polarization. However, as far as we know, this ultra-fast dipolar relaxation has thus far not been explained in a satisfactory way. We report similar ultra-fast dipolar relaxation of proton spins in naphthalene due to the photo-excited triplet spin of pentacene and propose a three-step mechanism that explains such ultra-fast dipolar relaxation by ground state electron spins as well as by photo-excited triplet spins: nuclear spin diffusion transfers nuclear dipolar order-that is nuclear dipolar energy-spatially to near the electron spins. Flip-flop transitions between nuclear spins near the electron spins convert this dipolar energy into electron-nuclear interaction energy. Finally electron spin-lattice relaxation or decay of the triplet spin transfers the latter type of energy to the lattice. We will show that this mechanism quantitatively explains the observed dipolar relaxation rate. The proposed mechanism is expected to contribute to dipolar relaxation in any spin system containing more than one spin species. It tends to create a stationary state, in which all dipolar interactions are combined in a single energy reservoir described by a single spin temperature. As an example we suggest that the addition of a relaxation agent in samples used for DNP may significantly accelerate the relaxation of the dipolar energy of the polarization agent, and as a result could possibly reduce the contribution of thermal mixing (TM) to DNP.
在动态核极化（DNP）的典型条件下——温度约为1K或更低，磁场约为3T或更高——极化剂会使核偶极序的弛豫速度比核极化快四个数量级。然而，据我们所知，这种超快的偶极弛豫至今尚未得到令人满意的解释。我们报告了由于并五苯的光激发三重态自旋导致萘中质子自旋出现类似的超快偶极弛豫现象，并提出了一种三步机制，该机制通过基态电子自旋以及光激发三重态自旋来解释这种超快偶极弛豫：核自旋扩散将核偶极序——即核偶极能量——在空间上转移到电子自旋附近。电子自旋附近的核自旋之间的翻转跃迁将这种偶极能量转化为电子 - 核相互作用能。最后，电子自旋 - 晶格弛豫或三重态自旋的衰减将后一种类型的能量转移到晶格。我们将表明，该机制定量地解释了观察到的偶极弛豫速率。所提出的机制有望对任何包含不止一种自旋种类的自旋系统中的偶极弛豫做出贡献。它倾向于创建一个稳态，其中所有偶极相互作用都合并到一个由单一自旋温度描述的单一能量库中。例如，我们认为在用于DNP的样品中添加弛豫剂可能会显著加速极化剂偶极能量的弛豫，结果可能会减少热混合（TM）对DNP的贡献。
