Multiple-Pulse NMR Experiments

Abstract

Multiple-pulse experiments have been applied to nuclear spins since the early days of NMR, beginning with the two pulse Hahn-echo [1], followed by three pulse echoes and the Carr-Purcell sequence [2], being the first cyclic multiple-pulse experiment at all. Later on modifications like the Gill-Meiboom [3] modification have been designed in order to reduce the influence of pulse errors which often have an accumulative effect in multiple-pulse sequences. Although these pulse sequences were first applied to liquids, it was soon realized, that similar pulse sequences could be applied to solids as well [4–8]. Even though only partial refocusing of the initial state was achieved, substantial insight into the spin dynamics of solids was gained in these experiments. Modified Carr-Purcell sequences applied to solids by Ostroff and Waugh [4] and by Mansfield and Ware [5] were unexpectedly successful in achieving a considerably lengthened decay of transverse magnetization. This effect contradicts the spin temperature hypothesis and was unexpected at first sight. Much effort was applied in designing more and more efficient multiple-pulse sequences. Since these sequences were promising in achieving a long transverse decay, but leaving resonance offset and chemical shift interactions effective, high resolution spectra in solids were expected. This development culminated in the four-pulse sequence by Waugh, Huber and Haeberlen [9], which is the most efficient basic multiple-pulse sequence up to date. Many modifications of this sequence have been proposed recently for correcting pulse errors.