Wide-Band NMR Spectroscopy with Frequency-Modulated “Chirp” Pulses
Frequency-modulated (“chirp”) pulses can be used very effectively to obtain adiabatic inversion over large frequency bandwidths and in the presence of B1 inhomogeneity. If a hard non-selective detection pulse is applied after partial recovery, an asymmetric response as a function of offset is obtained due to the fact that different spins are not inverted simultaneously during adiabatic passage. To preclude this effect and to excite transverse magnetization with negligible phase dispersion over large spectral widths, we propose to use an echo sequence composed of two chirp pulses. The initial л/2 pulse (“adiabatic half-passage”) causes a phase dispersion which is eliminated by a subsequent chirp refocusing pulse. The implications of longitudinal and transverse relaxation during both the inversion and echo sequence are investigated.