Wenckebach W Tom, Quan Yifan
National High Magnetic Field Laboratory, University of Florida, Gainesville, FL, USA; Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
Paul Scherrer Institute, CH-5232 Villigen, Switzerland.
J Magn Reson. 2021 May;326:106948. doi: 10.1016/j.jmr.2021.106948. Epub 2021 Feb 26.
The spectrum of the electron spin-spin interactions largely determines which mechanism is responsible for the growth of the nuclear spin polarization in dynamic nuclear polarization (DNP). When electron spin-spin interactions are weak and their spectrum is narrow, the solid effect (SE) dominates the process. When they are stronger, the cross effect (CE) and thermal mixing (TM) come into play. Then a narrow spectrum favours the CE-that is an exchange of electron Zeeman energy with the nuclear spins-and a broad spectrum also TM-that is an exchange of electron spin-spin interaction energy with the nuclear spins. Moreover, the spectrum of the electron spin-spin interactions critically determines the rate of spectral diffusion of electron spin polarization across the electron spin resonance (ESR) line, and the associated conversion of electron Zeeman energy into electron spin-spin interaction energy. This way electron spin-spin interactions indirectly influence the DNP process. The present work describes Monte Carlo simulations of the spectrum of these interactions for approximately spherical radicals in glasses and analytical approximations of the simulation results. As an example application expressions for the relative strengths of the energy flows due to the CE and TM are derived.
电子自旋-自旋相互作用的频谱在很大程度上决定了在动态核极化（DNP）中哪种机制导致核自旋极化的增长。当电子自旋-自旋相互作用较弱且其频谱较窄时，固体效应（SE）主导该过程。当它们较强时，交叉效应（CE）和热混合（TM）开始起作用。此时，窄频谱有利于CE（即电子塞曼能量与核自旋的交换），宽频谱也有利于TM（即电子自旋-自旋相互作用能量与核自旋的交换）。此外，电子自旋-自旋相互作用的频谱关键地决定了电子自旋极化在电子自旋共振（ESR）谱线上的频谱扩散速率，以及电子塞曼能量向电子自旋-自旋相互作用能量的相关转换。通过这种方式，电子自旋-自旋相互作用间接影响DNP过程。本工作描述了玻璃中近似球形自由基的这些相互作用频谱的蒙特卡罗模拟以及模拟结果的解析近似。作为一个示例应用，推导了由于CE和TM引起的能量流相对强度的表达式。
