Multi-frequency resonances in pure multiple-pulse NQR

We have observed multi-frequency resonances in a system with a spin 3/2 irradiated simultaneously by by a multiple-pulse radiofrequency sequence and a low frequency field swept in the range 0 divided by 80 kHz. The theoretical description of the effect is presented Using both the rotating frame approximation and the Floquet theory. Both approaches give indentical results at the calculation of the resonance frequencies, transition probabilities and shifts of resonance frequency. The calculated magnetization vs. the frequency of the low-frequency field agrees with the obtained experimental data.