Spin-Diffusion

In this presentation we are entirely concerned with transport of proton polarisation, in a sample characterised by the following: :

Under these conditions, the individual spins continually exchange energy with each other through the energy conserving flip-flop transitions of the Dipolar Hamiltonian. This provides a means by which polarisation differences in a sample can be evened out on a timescale which is more rapid than relaxation.

In recent years, it has been shown that these spin-flips are deterministic in character, and that restricted spin-systems can be described quantum mechanically. However, for the samples and timescales in volved in this work we are in the regime for which a thermodynamic approach is more appropriate.

If a non-uniform polarisation profile can be created the evolution of the NMR signal can be used to deduce the Spin-Diffusion behaviour. If the Spin-Diffusion coefficients are known the dimensions of the heterogeneities can be deduced (and vice versa).

Spin-Diffusion also couples the different regions in a heterogeneous solid, such that the observed spin-lattice relaxation behaviour differs from the intrinsic relaxation behaviour of the regions.


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