Two-pulse frequency-hopped excitation

Abstract

The use of frequency-hopped pulse pairs and pulse pair trains for exciting NMR signals is introduced. Pairs of pulses, each pair lasting one nutation period about the effective field in the rotating frame, can be used to efficiently tip nuclear spin magnetization. In the limit where the frequency difference between the partners in a pair is large, the magnetization dynamics mimic those of an ordinary rectangular rf pulse with rf amplitude scaled down by a factor of π/2. When the amplitude of the applied rf begins to approach the offset frequency however, the excitation bandwidth broadens dramatically in a manner reminiscent of a composite pulse. The effects of the frequency-hopped pulse pairs is described with analytical calculations and numerical simulations, and are shown to agree well with experiment.