Bulk nuclear spin magnetization

ONP (Fig. 3) in organic molecular crystals can be described as a three-step process (1) optical creation of electronic spin polarization in (transient) excited triplet states (2) polarization transfer to nearby nuclear spins and (3) accumulation of bulk nuclear spin magnetization in the (diamagnetic) ground state via spin-diffusion processes. Highly detailed theoretical descriptions and modeling of ONP processes observed in many systems can be found in the reviews mentioned above thus, only a brief overview is provided here. 

Thermal equilibrium produces a Boltzmann distribution between these energy levels and produces the bulk nuclear magnetization of the sample through the excess population which for a sample containing a total of N spins is -Ny/l BQllkT. For example for Si in an applied magnetic field of 8.45 T the excess in the... 

The chemical shift of a nuclear spin is a tensorial quantity. Its value depends on the orientation of the electronic distribution about the nucleus with respect to the external magnetic field. In a liquid, due to the rapid molecular motions, this interaction is averaged to zero and the observed chemical shift is the trace of the tensor. In contrast, in a powder, in the absence of motions, all the orientations have the same probability and the signal obtained for each carbon is the sum of the elementary chemical shifts corresponding to the different orientations. When local motions occur in the bulk below 7g, they usually induce a partial averaging of the chemical shift anisotropy. 

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