Other Non-Boltzmann Distributions

Saturation leads to equalization in the populations of the energy levels, contrary to the Boltzmann distribution. On the other hand, a number of NMR techniques can be employed to increase the population difference well beyond that given by the Boltzmann distribution. In some instances it is convenient to retain the formalism of the Boltzmann relation by defining a spin temperature Ts that satisfies Eq. 2.19 for a given ratio n /na. For times much less than T, it is meaningful to have Ts = T, the macroscopic temperature of the sample, because the spin system and lattice do not interact in this time frame.Viewed in this way, saturation 

Discussion of the these three methods is outside the scope of this book, but in later chapters we consider other methods for producing much less dramatic non-Boltzmann distributions. By using rf irradiation to alter spin populations, the nuclear Overhauser effect results in signal enhancement (Chapters 8 and 10). Several techniques use pulse sequences to transfer polarization from nuclei with large y to nuclei with small y in solids (Chapter 7) and liquids (Chapters 9 and 12), hence to provide significant signal enhancement. 

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