Improving resolution in fast rotating-frame experiments.

The rapid rotating-frame technique allows significant reduction in data-acquisition time compared with the two-dimensional method by stroboscopic observation of the nuclear magnetization during its evolution in the rotating frame. A onefold reduction in the dimensionality of the original rotating-frame experiment is achieved by using a train of strong radiofrequency pulses separated by short acquisition windows. The penalty for shortening experimental time is a reduction in spectral resolution compared with the two-dimensional method due to relaxation of transverse magnetization components during the observation windows. A variant of the rapid-rotating frame technique for improving spectral resolution based on undersampling and self-phase encoding is presented. An M-fold resolution improvement requires M experiments, thus, making possible a tradeoff between spectral resolution and experimental time. The technique was applied for spatial localization of quadrupole nuclei in powder solids, and resolution improvement is demonstrated on one- and two-dimensional NQR images.