Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
J Chem Phys. 2010 Jun 7;132(21):214504. doi: 10.1063/1.3428665.
Electron-electron double resonance pulsed electron paramagnetic resonance (EPR) at 95 GHz (3.3 T) is used to follow the dynamics of the electron spin polarization during the first stages of dynamic nuclear polarization in solids. The experiments were performed on a frozen solution of Gd(+3) (S=7/2) in water/glycerol. Focusing on the central vector -1/2 --> vector +1/2 transition we measured the polarization transfer from the Gd(3+) electron spin to the adjacent (1)H protons. The dependence of the echo detected EPR signal on the length of the microwave irradiation at the EPR "forbidden" transition corresponding to an electron and a proton spin flip is measured for different powers, showing dynamics on the microsecond to millisecond time scales. A theoretical model based on the spin density matrix formalism is suggested to account for this dynamics. The central transition of the Gd(3+) ion is considered as an effective S = 1/2 system and is coupled to (1)H (I = 1/2) nuclei. Simulations based on a single electron-single nucleus four level system are shown to deviate from the experimental results and an alternative approach taking into account the more realistic multinuclei picture is shown to agree qualitatively with the experiments.
电子-电子双共振脉冲电子顺磁共振（EPR）在 95GHz（3.3T）下用于跟踪固态动态核极化最初阶段电子自旋极化的动力学。该实验是在水/甘油中的 Gd(+3)（S=7/2）的冷冻溶液上进行的。研究人员关注中央向量-1/2-->向量+1/2 跃迁，测量了 Gd(3+)电子自旋到相邻 (1)H 质子的极化转移。在不同功率下，测量了与电子和质子自旋翻转对应的 EPR“禁止”跃迁的微波辐照长度对回波检测 EPR 信号的依赖关系，显示出微秒到毫秒时间尺度上的动力学。提出了一个基于自旋密度矩阵形式主义的理论模型来解释这种动力学。将 Gd(3+)离子的中央跃迁视为有效的 S=1/2 系统，并与 (1)H（I=1/2）核耦合。基于单电子-单核四级体系的模拟显示与实验结果存在偏差，而考虑到更实际的多核图像的替代方法则定性上与实验结果一致。
