Relaxation, diffusion-limited, rate defined

When using the temperature dependence of the nuclear spin-spin or nuclear spin-lattice relaxation rate to study molecular motion, as is the case with the surface diffusion we are dealing with here, there exist soolled strong and weak collision limits. Different mathematical relationships are needed to describe these limits. Consider the nuclear spin-spin relaxation rate (1 / T2) as measured by a conventional Hahn-echo pulse sequence, and suppose that Aa> is the amplitude of the local field fluctuation responsible for relaxation. Also assume that r is the correlation time for the motion, say a jump, which causes the local field to fluctuate. The strong collision limit is defined such that... 

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