Long-Lived Spin States in Nuclear Magnetic Resonance

核磁共振是一种技术，它利用了这样一个事实，即许多原子核充当微小的无线电发射器，以精确定义的频率发射无线电信号，可以由仔细调整的探测器检测到。信号的频率和强度取决于样品所在的磁场：磁场越高，频率越高，信号越强。在核磁共振实验中，首先通过将样品置于强磁场中一段时间来磁化原子核。然后，一系列射频脉冲被施加到样本上，然后样本发出无线电波，可以在无线电接收器中检测到。发射波的模式取决于原子核在脉冲序列中经历的情况。一个有用的特征是，原子核可以记住在无线电信号发出前几秒钟发生在它们身上的事情。这种记忆特性使人能够通过核磁共振跟踪运动，如化学反应、分子的随机置换以及血液和其他液体的流动。直到最近，原子核的记忆时间还被认为是被研究物质的一个固定性质，这不能通过做实验的方式来显著改变。然而，我们的团队在2004年表明，对于某些物质，通过应用特定的射频脉冲序列，记忆时间可以延长10倍或更多。我们已经演示了一种新的现象，现在称为长寿命自旋态(LLSS)。在这个项目中，我们将试图更好地理解LLSS现象，并学习如何将其应用于运动过程的研究。从长远来看，这将为科学家、工程师和医生提供了解化学物质的行为和运动的新工具。

Nuclear Magnetic Resonance (NMR) is a technique which uses the fact that the nuclei of many atoms act as tiny radiotransmitters, emitting radio signals at precisely-defined frequencies, which can be detected by a carefully-tuned detector. The frequencies and strengths of the signals depend on the magnetic field in which the sample is placed: the higher field, the higher the frequency, and the stronger the signals. In an NMR experiment, the nuclei are first magnetized by placing a sample in a strong magnetic field for some time. A sequence of radiofrequency pulses is then applied to the sample, which then emits radiowaves which can be detected in the radio receiver. The pattern of emitted waves depends on what the nuclei experienced during the pulse sequence. One useful feature is that the nuclei can remember what happened to them some seconds before the radiosignals are emitted. This memory property allows one to track movements such as chemical reactions, the random displacement of molecules, and the flow of blood and other fluids by NMR. Until recently, the memory time of the atomic nuclei was thought to be a fixed property of the substance under study, which could not be changed significantly by the way one does the experiment. However, our group showed in 2004 that for some substances the memory time could be extended by a factor of 10 or more, by applying a certain sequence of radiofrequency pulses. We had demonstrated a new phenomenon which is now called long-lived spin states (LLSS). In this project, we will try to understand the LLSS phenomenon better and learn how to apply it to the study of motional processes. In the long term, this will provide scientists, engineers and doctors with new tools for understanding the behaviour and motion of chemical substances.