Cross-relaxation effects in stimulated-echo-type PGSE NMR experiments by bipolar and monopolar gradient pulses

Exchange of longitudinal spin polarization by dipolar cross relaxation betw een nonequivalent spins results in a modulation of the stimulated echo sign al on increasing the encoding/decoding delays and in a multiexponential dec ap on increasing the diffusion time. These artifacts are suppressed by 180 degrees pulses inserted in the middle of the gradient encoding/decoding per iods. The efficiency of the gradient encoding is preserved if bipolar gradi ent pulses are used instead of monopolar pulses. The behavior of the differ ent pulse sequences is demonstrated by F-19 PGSE NMR experiments in a lyotr opic liquid crystal in both isotropic micellar and oriented nematic phases. (C) 2000 Academic Press.