Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics
Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics
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DOI：
10.1063/1.3564920
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发表时间：
2011-03-28
影响因子：
4.4
通讯作者：
Griffin, Robert G.
中科院分区：
文献类型：
--
作者：
Hu, Kan-Nian;Debelouchina, Galia T.;Griffin, Robert G.
微波驱动的动态核极化（DNP）是一个过程，其中存在于电子自旋库中的大极化被转移到核，从而增强NMR信号强度。在固体相变中，有三种机制介导这种传递-固体效应（SE）、交叉效应（CE）和热混合（TM）。从历史上看，这些机制已经在理论上使用热力学参数和平均自旋相互作用进行了讨论。然而，SE和CE也可以用由少量自旋组成的系统进行量子力学建模，并且结果为涉及TM的计算提供了基础。在SE的情况下，单个电子-核自旋对足以解释极化机制，而CE需要两个电子和一个核自旋的参与，并且可以用于理解使用双自由基极化剂观察到的改进的DNP增强。计算建立了电子顺磁共振（EPR）和核磁共振（NMR）频率和微波辐射频率之间的关系，必须满足通过SE或CE的极化转移。特别地，如果delta，Delta < omega（0 I），其中delta和Delta分别是EPR谱的均匀线宽和非均匀宽度，我们验证当omega（M）= omega（0 S）+/- omega（0 I）时发生SE，其中omega（M）、omega（0 S）和omega（0 I）分别是微波、EPR和NMR频率。或者，当Delta > omega（0 I）> Delta时，CE主导偏振转移。当Ω（0 S1）-Ω（0 S2）= Ω（0 I）和Ω（M）类似于Ω（0 S1）或Ω（0 S2）时，这种双电子过程被优化，其中Ω（0 S1）和Ω（0 S2）是两个电子的EPR拉莫尔频率。利用这些匹配条件，我们计算了密度算符从电子塞曼阶到核塞曼阶的演化。结果提供了深入了解微波辐射场，外部磁场，电子-电子和电子-核相互作用对DNP增强的影响。(C)2011年美国物理学会。[doi：10.1063/1.3564920]
Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer-the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron-nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if delta, Delta < omega(0I), where delta and Delta are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when omega(M) = omega(0S) +/- omega(0I), where omega(M), omega(0S) and omega(0I) are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Delta > omega(0I) > delta, the CE dominates the polarization transfer. This two-electron process is optimized when omega(0S1) - omega(0S2) = omega(0I) and omega(M) similar to omega(0S1) or omega(0S2), where omega(0S1) and omega(0S2) are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron-electron and electron-nuclear interactions on DNP enhancements. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3564920]