BEHAVIOR OF LONGITUDINAL SPIN ORDERS IN NMR MEASUREMENTS OF SELF-DIFFUSION COEFFICIENTS USING RADIOFREQUENCY FIELD GRADIENTS

Weakly coupled spin 1/2 systems of the type AX and AX, can be easily b rought into states of longitudinal order represented (in terms of oper ator products) by 2I(z)(A)I(z)(X) and 4I(z)(A)I(z)(X)I(z)(X'), respect ively. It proves possible to follow the evolution of these states unde r translational diffusion processes through a simple experiment involv ing two identical radiofrequency field gradient pulses separated by an interval allowing diffusion to take place. Relevant theory and experi mental verifications are provided. The main conclusion is that, for a part of the decay, the effective gradient is multiplied by a factor of two in the case of a two spin order (2I(z)(A)I(z)(X)) and a factor of three in the case of a three spin order (4I(zz)(AX)I(z)(X')). These p roperties can be employed for accessing to self-diffusion coefficients , smaller by factors of four and nine respectively, as compared with t hose measured by conventional methods. (C) 1996 American Institute of Physics.