  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.
