STRETCHED ADIABATIC PULSES FOR BROAD-BAND SPIN INVERSION

The various modes of adiabatic fast passage are analyzed with regard t o their effectiveness for spin inversion. For all the existing schemes , the critical point is the central part of the sweep where the adiaba tic condition is most stringent. In this region, the sweep is approxim ately linear and the minimum radiofrequency intensity is determined by the adiabatic requirement. It is demonstrated that the effective band width for spin inversion may be considerably improved by greatly exten ding this region of linear frequency sweep so as to encompass the enti re range of chemical shifts, subject to the constraint imposed on the pulse duration, Then the experimental conditions (radiofrequency level , sweep rate, and adiabaticity) are essentially the same for all spins within the effective bandwidth. These new ''stretched'' adiabatic pul ses approach the optimum achievable bandwidth for spin inversion for a given radiofrequency level and offer substantial improvements when us ed for broadband heteronuclear decoupling, Residual imperfections can be compensated by suitable phase cycles and supercycles. When these pu lses are adjusted for very nide effective bandwidths, cycling sideband s can be appreciable, but the method is sufficiently flexible that it can be tailored to suit almost any practical application, trading off a reduced bandwidth for lower sideband intensities. (C) 1995 Academic Press, Inc.