Second-Order Echo Signals in Two-Pulse Delayed Nutation

Nonstationary magnetic nutation signals of the second-order echo that occur during the effect of a resonance radiofrequency pulse of length t ₂ on a two-level inhomogeneously broadened spin system preliminarily excited by a pulse of length t ₁ < t ₂ have been studied theoretically. It has been found that in contrast to the delayed nutation echo whose formation is attributed to the restoration of the longitudinal magnetization that arose by the end of the first pulse, these signals result from the reversal in time of three effective signals of free precession generated after the termination of the first pulse. The theoretical results are in good agreement with the experimental data obtained in NMR of protons in glycerin. A conclusion on the validity of the theorem of coherent transient processes occurring in two-level systems in the presence of an exciting field is drawn.