The structure of coupled electron spin systems is of fundamental interest to many applications, including dynamic nuclear polarization (DNP), enhanced nuclear magnetic resonance (NMR), the generation of electron spin qubits for quantum information science (QIS), and quantitative studies of paramagnetic systems by electron paramagnetic resonance (EPR). However, the characterization of electron spin coupling networks is nontrivial, especially at high magnetic fields. This study focuses on a system containing high concentrations of trityl radicals that give rise to a DNP enhancement profile of ¹H NMR characteristic of the presence of electron spin clusters. When this system is subject to selective microwave saturation through pump–probe ELectron DOuble Resonance (ELDOR) experiments, electron spin hyperpolarization is observed. We show that the generation of an out-of-equilibrium longitudinal dipolar order is responsible for the transient hyperpolarization of electron spins. Notably, the coupled electron spin system needs to form an AX-like system (where the difference in the Zeeman interactions of two spins is larger than their coupling interaction) such that selective microwave irradiation can generate signatures of electron spin hyperpolarization. We show that the extent of dipolar order, as manifested in the extent of electron spin hyperpolarization generated, can be altered by tuning the pump or probe pulse length, or the interpulse delay in ELDOR experiments that change the efficiency to generate or readout longitudinal dipolar order. Pump–probe ELDOR with selective saturation is an effective means for characterizing coupled electron spins forming AX-type spin systems that are foundational for DNP and quantum sensing.

中文翻译：

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偶极级诱导电子自旋超极化


耦合电子自旋系统的结构对许多应用具有根本意义，包括动态核极化（DNP）、增强核磁共振（NMR）、用于量子信息科学（QIS）的电子自旋量子位的生成以及顺磁的定量研究电子顺磁共振（EPR）系统。然而，电子自旋耦合网络的表征并非易事，尤其是在高磁场下。本研究重点关注含有高浓度三苯甲基自由基的系统，该系统会产生 ¹ H NMR 的 DNP 增强曲线，这是电子自旋簇存在的特征。当该系统通过泵浦探针电子双共振（ELDOR）实验进行选择性微波饱和时，会观察到电子自旋超极化。我们表明，不平衡纵向偶极有序的产生是电子自旋瞬时超极化的原因。值得注意的是，耦合电子自旋系统需要形成类 AX 系统（其中两个自旋的塞曼相互作用的差异大于它们的耦合相互作用），以便选择性微波辐射可以产生电子自旋超极化的特征。我们表明，偶极有序的程度（表现为电子自旋超极化产生的程度）可以通过调整泵浦或探针脉冲长度或 ELDOR 实验中的脉冲间延迟来改变，从而改变产生或读出纵向偶极有序的效率。具有选择性饱和的泵浦探针 ELDOR 是表征形成 AX 型自旋系统的耦合电子自旋的有效方法，该系统是 DNP 和量子传感的基础。