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. Introduction One of the most effective and promising high-resolution nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) techniques for the study of solids is a multiple-pulse radiofrequency (RF) action [1,2]. The multiple-pulse methods allow one to remove dipolar broadening from a resonance line in solids thus increasing by several orders the sensitivity of the NMR and NQR spectroscopy in the study of weak interaction. These methods are very effective in the study of the spin lattice relaxation processes due to a slow atomic motion. Usually the theoretical description of multiple-pulse experiments both in NMR [3] and NQR [4] is based on the construction of the effective time-independent Hamiltonian by using the conditions for periodicity and cyclicity of the pulsed action . Then the dynamics of a spin system subjected by pulsed RF fields is presented in an equivalent form as the motion of nuclear spins in a constant effective field He [5]. The magnitude and direction of this effective field are determined by parameters of the multiple-pulse sequence. An experimental measurement of the value of the effective field is important for the confirmation