Nuclear Spins far from Equilibrium

There are hundreds of nuclear magnetic resonance (NMR) methods applied today, ranging from methods for the imaging of anatomic structure (MRI) to methods for determining molecular structure and dynamics in solution and in the solid state. However most of these techniques employ nuclear spin systems which remain very close to a state of thermal equilibrium with respect to the molecular environment. 

It is now possible to prepare substances in which the nuclear spin systems are very far from thermal equilibrium. Such systems may, in some cases, give rise to NMR signals which are highly enhanced with respect to signals obtained under routine conditions. The enhancement factor may be as large as 10^5, facilitating entirely new classes of NMR applications such as the imaging of metabolism in vivo. There are also modes of non-equilibrium nuclear spin order which are non-magnetic and do not give rise to NMR signals, but which are relatively long-lived, allowing the non-equilibrium state to be maintained for a relatively long time. Methods exist for extracting hyperpolarized spin order from such non-magnetic non-equilibrium states, on demand. In some cases, highly non-equilibrium nuclear spin order gives rise to non-magnetic effects, such as a change in the dielectric constant of the material. 

I will review the types of non-equilibrium spin order that exist, describe how they are prepared, how they may be interconverted, and how they may be applied.