Stretched adiabatic pulses for broadband spin inversion

The various modes of adiabatic fast passage are analyzed with regard to their effectiveness for spin inversion. For all the existing schemes, the critical point is the central part of the sweep where the adiabatic condition is most stringent. In this region, the sweep is approximately linear and the minimum radiofrequency intensity is determined by the adiabatic requirement. It is demonstrated that the effective bandwidth for spin inversion may be considerably improved by greatly extending this region of linear frequency sweep so as to encompass the entire 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 pulses approach the optimum achievable bandwidth for spin inversion for a given radiofrequency level and offer substantial improvements when used for broadband heteronuclear decoupling, Residual imperfections can be compensated by suitable phase cycles and supercycles. When these pulses are adjusted for very nide effective bandwidths, cycling sidebands 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.