Coherent NMR responses in spin systems with complex spectral structure

Abstract

We show that for nonresonant excitation of a quantum system, with an NMR spectrum consisting of two inhomogeneously broadened lines, by a combination of an extended and a short delta-shaped pulse, in the free-precession and spin-echo signals after the second pulse we observe the effect of doubling of the additional emission signals. We establish that the nature of this effect is due to zero beats arising when the variable oscillation frequency of the magnetic moments of the material matches the detuning of the pulse carrier frequency from the resonant frequencies of the spectral lines. Within the theory obtained, we propose a formula which we can use, if we know the times at which the additional emission signals arise, to very accurately determine the frequency shift of the lines in the complex spectrum of the material. The theoretical results agree well with experimental data on generation of multiple NMR signals in toluene, the spectrum of which consists of two non-overlapping lines.