Sensitivity enhancement of solid-state NMR spectra of half-integer spin quadrupolar nuclei using hyperbolic secant pulses: Applications to spin-5/2 nuclei

The use of hyperbolic secant, HS, pulses to obtain NMR signal enhancements for the central transition of spin-5/2 quadrupolar nuclei in solids is explored. By inverting the populations of nuclear-spin energy levels prior to applying a selective-observe pulse, enhancements near the theoretical maximum are obtained for the central Al-27 NMR transition in a single crystal of alpha-Al2O3. The experimental enhancements obtained using HS inversion pulses for powder samples are compared to those obtained using the double-frequency sweep method. For every sample investigated, the HS enhancement factors are equal to or superior to those previously reported using other enhancement techniques. (c) 2006 Elsevier B.V. All rights reserved.