Multi-photon resonances in pure multiple-pulse NQR
  We have observed multi-photon resonances in a system with a spin 3/2 irradiated simultaneously by a multiple pulse radiofrequency sequence and a low frequency field swept in the range 0-80 kHz. The used excitation scheme allowed us to measure the effective field of the radiofrequency sequence. A peculiarity of this scheme is that the intensity of the resonance lines decreases slowly with the mode number. 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 well with the obtained experimental data. The multi-frequency spectra give a way for studying slow atomic motion in solids. 
G. B. Furman, G.E Kibrik, and A.Yu. Polyakov