Line shapes, short relaxation

The faithful representation of the shape of lines broadened greatly by dipolar and, especially, quadrupolar interactions often requires special experimental techniques. Because the FID lasts for only a very short time, a significant portion may be distorted as the spectrometer recovers from the short, powerful rf pulse. We saw in Section 2.9 that in liquids a 90°, t, 180° pulse sequence essentially recreates the FID in a spin echo, which is removed by 2r from the pulse. As we saw, such a pulse sequence refocuses the dephasing that results from magnetic field inhomogeneity but it does not refocus dephasing from natural relaxation processes such as dipolar interactions. However, a somewhat different pulse sequence can be used to create an echo in a solid—a dipolar echo or a quadrupolar echo—and this method is widely employed in obtaining solid state line shapes (for example, that in Fig. 7.10).The formation of these echoes cannot readily be explained in terms of the vector picture, but we use the formation of a dipolar echo as an example of the use of the product operator formalism in Section 11.6. 

Both the a-Si D,F and the alloy a-SiGe D,F show a small ( 5%) component characterized by a broad line shape and a very short Ti- The preliminary Tx for the a-Si D,F sample (indicated by solid diamonds in Fig. 4) suggest that this component is closely related to the P-D2 relaxation centers, whose theoretical Tx are given by the dashed line. This component probably arises from dilute or effectively-dilute P-D2 relaxation centers... 

It is not possible to lay down any universal rules since receptivities vary widely and relaxation times may be very long, especially for Li, or extremely short. Where the relaxation times are not too long, data are easy to obtain in the FT mode and the disadvantages of shorter relaxation times and broad lines can often be overcome to a great extent since short acquisition times and rapid pulse repetition rates are permissible, especially if some line-shape distortion can be tolerated. For maximum use to be made of such techniques access to memory sizes less than IK must be possible. 

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