Zur Definition von Relaxationszeit und Korrelationszeit in magnetischen Resonanzexperimenten

Kubo's general definition of relaxation and correlation times in magnetic spin systems, applicable to non-exponential processes, is evaluated for several non-exponential relaxation and correlation functions known from nmr experiments. The new definition eliminates the arbitrary factors usually encountered in the time constants of nonexponential irreversible processes. For the correlation time of the well-known “translational two-spin model”, three different values are used in the literature; our definition leads toτ _t =1/5 d²/D (d=distance of closest approach between the two spins,D=diffusion coefficient of the related molecules), which is an intermediate value to the conventional abbreviationsτ _t =1/5d ²/D andτ _t =1/6d ²/D.