Dynamic Orientation of Nuclei at Low Temperatures

A study has been made of the mechanisms of dynamic polarization of nuclei in organic materials at temperatures below 1 K and in magnetic fields up to 50 kG. After a summary of the mechanisms of dynamic polarization, the experimental results on 1,2-propanediol, doped with paramagnetic Crv complexes, are compared quantitatively with a spin temperature model of Borghini, in which it is assumed that the nuclear polarization is obtained via thermal contact with a dynamically cooled electron spin-spin interaction reservoir. The strong influence of this latter reservoir was apparent from the transient behavior of different nuclear spin systems contained in the same sample. The abovementioned spin temperature model has the unique property of not using the simplifying "high-temperature approximation" and it was the first time that this model has been found to be in reasonable agreement with experimental data in the 100 mK region. Furthermore, dynamic polarization experiments with the free radical BDPA are reviewed. These measurements show clearly the contributions of different mechanisms of dynamic polarization. The results are found to be in agreement with formulas derived from the spin temperature theory after introduction of some simplifying assumptions in order to be able to deal with low temperatures.