Effect of Motion on an Echo

An implicit assumption underlies our discussion of echoes thus far, namely that dephasing and rephasing occur under opposite but otherwise precisely identical conditions. If a molecule moves in such a way that it experiences different values of the static field during the two phases, refocusing of the signal is incomplete. This potential problem can be turned to advantage by controlling the variables in such a way that molecular motion, either bulk flow or molecular diffusion, can be measured by NMR. 

Molecular diffusion can also be determined by NMR, as we describe in the next section. 

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