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In a classic paper, Solomon showed that the presence of inhomogeneous, first-order quadrupolar interactions leads to the formation of extra "allowed" spin echoes in nuclei for which the spin I = 5 2 . We demonstrate (both theoretically and experimentally) that shifting the rf phase of the second pulse by 90° (in a spin- 5 2 system) enhances the extra allowed echoes by a factor of almost 5 in amplitude over the unshifted case. Using the density-matrix formulation (and assuming no magnetic inhomogeneities), we have derived, for a 90° phase shift, the amplitude and shape dependence on the second-pulse turning angle of the 3 2 τ , 2 τ , and 3 τ echoes. Experimental echo amplitudes and shapes (for both 0° and 90° phase shifts) were obtained, at room temperature, on I 127 in a fused sample of KI, and these show good agreement with the calculations. Because of the enhancement, this technique affords a much easier separation of the respective distributions arising from the 3 2 ↔ 5 2 and from the 1 2 ↔ 3 2 satellite transitions than is possible in the unshifted case. Another feature of the phase-shifted case is that, in favorable circumstances, the 3 τ echo may be observed although the 2 τ echo is obscured by the receiver recovery time. Preliminary data on quadrupole distributions at Al sites in NiAl and Au Al 2 intermetallic compounds are presented. Spin-spin relaxation of the satellite transitions is noted. Results expected for systems which have spin values other than I = 5 2 (both integral and half-integral) are mentioned.
Physical Review – American Physical Society (APS)
Published: May 15, 1969
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