电子-核接触和微波饱和在热混合DNP中的作用
On the role of electron-nucleus contact and microwave saturation in Thermal Mixing DNP
摘要：
我们探讨了多种物理场景下出现的动态核极化（DNP）通过热混合的假设下，高效的电子-电子相互作用的模型。当电子和核库也被假定为处于强热接触并且微波辐射使靶电子跃迁饱和时，即使辐射频率被设置为远离ESR线的中心（如Borghini已经观察到的），核极化的增强预计也相当高，并且典型的极化时间在向所述线的边界移动时减少。在核泄漏情况下，通过降低微波饱和度或电子与原子核之间的接触，可以获得更合理的行为。在这两种情况下，描述稳态核极化对辐照频率的依赖性的函数在边缘处变得更尖锐，并且建立速率在移动离共振时减小。如果就对核极化产生的影响而言，微波饱和度和电子-核接触的程度在性质上是相似的，则对电子极化具有完全不同的影响，这当然与饱和的有效性密切相关，并且在稳态下对两个自旋库之间的相互作用的大小几乎不敏感。不同的情况下的可能性进行了讨论，目前在文献中的实验数据的光，指出哪些方面是适当的考虑，哪些是不通过这里考虑的热混合DNP的赤纬。
英文摘要：
We have explored the manifold physical scenario emerging from a model of Dynamic Nuclear Polarization (DNP) via thermal mixing under the hypothesis of highly effective electron-electron interaction. When the electron and nuclear reservoirs are also assumed to be in strong thermal contact and the microwave irradiation saturates the target electron transition, the enhancement of the nuclear polarization is expected to be considerably high even if the irradiation frequency is set far away from the centre of the ESR line (as already observed by Borghini) and the typical polarization time is reduced on moving towards the boundaries of said line. More reasonable behaviours are obtained by reducing the level of microwave saturation or the contact between electrons and nuclei in presence of nuclear leakage. In both cases the function describing the dependency of the steady state nuclear polarization on the frequency of irradiation becomes sharper at the edges and the build up rate decreases on moving off-resonance. If qualitatively similar in terms of the effects produced on nuclear polarization, the degree of microwave saturation and of electron-nucleus contact has a totally different impact on electron polarization, which is of course strongly correlated to the effectiveness of saturation and almost insensitive, at the steady state, to the magnitude of the interactions between the two spin reservoirs. The likelihood of the different scenario is discussed in the light of the experimental data currently available in literature, to point out which aspects are suitably accounted and which are not by the declinations of thermal mixing DNP considered here.
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