Numerically optimized modulations for adiabatic pulses in surface nuclear magnetic resonance

Abstract

Adiabatic pulses, which provide an effective means of generating a large-amplitude nuclear magnetic resonance (NMR) signal in the presence of a heterogeneous magnetic field, have the potential to greatly improve the signal-to-noise ratio of the surface NMR experiment. To ensure efficient implementation of adiabatic pulses into the surface NMR framework, a numerically optimized modulation (NOM) approach is used to design adiabatic pulses specifically intended for application in surface NMR. The scenario in which the frequency response of the tuned transmit coil is used to modulate the current amplitude is considered. The performance of a NOM pulse is contrasted against two alternative adiabatic pulses (described by a linear frequency sweep and a hyperbolic tangent sweep) that are currently implemented with the existing hardware. The NOM approach provides equivalent excitation as the chirp and hyperbolic tangent pulse while shortening pulse durations and reducing power consumption. Furthermore, the NOM approach also provides sharp resolution and large signal amplitudes. Considerations for the design of the NOM adiabatic pulse for surface NMR are given, as well as a discussion about their implementation into the surface NMR experimental framework.