Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
J Chem Phys. 2012 Sep 14;137(10):104508. doi: 10.1063/1.4751021.
During dynamic nuclear polarization (DNP) at 1.5 K and 5 T, (129)Xe nuclear magnetic resonance (NMR) spectra of a homogeneous xenon/1-propanol/trityl-radical solid mixture exhibit a single peak, broadened by (1)H neighbors. A second peak appears upon annealing for several hours at 125 K. Its characteristic width and chemical shift indicate the presence of spontaneously formed pure Xe clusters. Microwave irradiation at the appropriate frequencies can bring both peaks to either positive or negative polarization. The peculiar time evolution of (129)Xe polarization in pure Xe clusters during DNP can be modelled as an interplay of spin diffusion and T(1) relaxation. Our simple spherical-cluster model offers a sensitive tool to evaluate major DNP parameters in situ, revealing a severe spin-diffusion bottleneck at the cluster boundaries and a significant sample overheating due to microwave irradiation. Subsequent DNP system modifications designed to reduce the overheating resulted in four-fold increase of (129)Xe polarization, from 5.3% to 21%.
在 1.5 K 和 5 T 的动态核极化 (DNP) 条件下，均匀的氙气/1-丙醇/三苯甲基自由基固体混合物的 (129)Xe 核磁共振 (NMR) 谱显示出一个单一的峰，由于 (1)H 核的邻位而变宽。在 125 K 下退火几个小时后，会出现第二个峰。其特征宽度和化学位移表明存在自发形成的纯 Xe 团簇。用适当频率的微波照射可以将两个峰分别极化到正或负。在 DNP 过程中，纯 Xe 团簇中 (129)Xe 极化的特殊时间演化可以用自旋扩散和 T(1)弛豫的相互作用来模拟。我们的简单球形团簇模型提供了一个敏感的工具，可以原位评估主要的 DNP 参数，揭示了在团簇边界处存在严重的自旋扩散瓶颈，以及由于微波照射导致的显著的样品过热。随后设计的 DNP 系统改进旨在减少过热，使 (129)Xe 极化从 5.3%增加到 21%。
