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 27Al NMR transition in a single crystal of α-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.