Spin dynamics and optimisation of dynamic nuclear polarisation at cryogenic temperatures

核磁共振是一种基于对样品的微弱磁化进行测量的技术，该磁化是由于外部强磁场与样品中携带磁矩的各种类型的核相互作用而产生的。外加磁场对原子核的磁矩产生的相互作用力倾向于使原子核在平行于外场方向的方向上排列。然而，热运动抵消了这种排列，由于外场和磁矩之间的相互作用力非常弱，具有平行排列的原子核的数量和沿相反(或反平行)方向排列的自旋数量之间只有很小的差异。这就是为什么核磁共振技术通常被认为不是高度敏感的原因。通过使用液氦将样品冷却到非常低的温度，可以减少热运动。其结果是，样品的磁化强度增加。未成对的电子具有磁矩，它与外部磁场的相互作用比质子强约3个数量级，其温度为1K，适度的外部磁场为3.5T，几乎完全与外部磁场的方向一致。在核磁共振术语中，这意味着电子是100%极化的。未配对的电子耦合到磁性活跃的核自旋，并有可能利用这种相互作用将电子极化转移到磁性活跃的原子核上。原则上，电子被用来使原子核在一个方向上排列。这个过程被称为动态核极化。这个过程的动力学可以用量子力学推导出来。然而，量子力学的数学公式意味着，如果假设一个由多个耦合核组成的系统，这个问题就很难解决。这一建议研究了获得多个耦合自旋系统的动力学信息的数学策略。此外，还测量了使模型计算有意义所需的系统参数。在第二步中，我们在理论上和之后的实验实施中研究了优化电子系统和核系统之间极化转移的策略。这个项目的主要目标是深入了解动态核极化的动力学，然后利用这些信息在更快的时间尺度上产生更高水平的核极化。该项目的成果将有利于核磁共振波谱和磁共振成像的许多应用，这些应用受到低灵敏度的限制。使用有效的DNP策略将有可能通过成像预极化分子的分布和代谢转换来产生高极化，用于分子动力学研究或医学诊断应用。

Nuclear Magnetic Resonance (NMR) is a technique that is based on the measurement of a weak sample magnetisation that arises from the interaction of a strong external magnetic field with various types of nuclei in the sample which carry magnetic moments. The interaction force generated by the external magnetic field on the magnetic moments of the nuclei tends to align the nuclei in an orientation parallel to the external field direction. However, thermal motion counteracts this alignment and since the interaction force between external field and magnetic moments is only very weak there is only a small difference between the population of nuclei that have a parallel alignment and the population of spins that are aligned in the opposite (or anti-parallel) direction. This is the reason why NMR techniques are usually considered to be not highly sensitive.The thermal motion can be reduced by cooling the sample to very low temperatures using liquid helium. As a consequence the sample magnetisation increases. Unpaired electrons, which possess a magnetic moment and which interact about 3 orders of magnitude stronger than protons with the external magnetic field are at a temperature of 1K and a modest external magnetic field of 3.5T almost fully aligned with the direction of the external field. In NMR terminology this means that the electrons are 100% polarised. Unpaired electrons couple to magnetically active nuclear spins and it is possible to use this interaction to transfer the electronic polarisation onto the magnetically active nuclei. In principle, the electrons are used to align the nuclei in one direction. This process is called dynamic nuclear polarisation. The dynamics of the process can be derived using quantum mechanics. However, the mathematical formulation of quantum mechanics means that the problem becomes difficult to be solved in case a system of many coupled nuclei is assumed. This proposal investigates mathematical strategies to obtain the dynamical information for systems of many coupled spins. Furthermore, the system parameters are measured that are needed to make the model calculations meaningful. In a second step strategies are investigated in theory and afterwards in experimental implementations to optimise the transfer of polarisation between the electron system and the nuclear system.The key objective of this project is to gain insight into the dynamics of dynamics nuclear polarisation and then use this information to generate higher levels of nuclear polarisation on an even faster time scale. The outcome of the project will benefit numerous applications of nuclear magnetic resonance spectroscopy and magnetic resonance imaging which are limited by the low sensitivity. Using efficient DNP strategies is will be possible to generate high polarisation for studies of molecular dynamics or also for applications of medical diagnostics by imaging the distribution and metabolic conversion of pre-polarised molecules.