Two-quanta progressive saturation: pulsed NMR of 23Na in single crystal sodium nitrate

Progressive saturation effects due to induction of two-quanta transitions in the quadrupolar-split levels of a spit, 3/2, (²³Na) nucleus were studied by monitoring changes occuring in the 1/2 → ? 1/2 magnetization after applying trains of identical pulses at various radiofrequencies and repetition rates. The variety of single- and double-quantum (DO) interconnected population effects observed in the experiments is explained by incorporating solutions of the ordinary two-level Bloch equations with those of the two-quanta, three-level Bloch-like equations (including off-resonance conditions) derived recently by Gold and Hahn. Small displacements of nuclear levels under DQ irradiation (analogous to the optical ac Stark shift) the cause rf power-dependent DQ frequency shifts are predicted for all spins I> 1, and explicit expressions are derived. Additional effects due to state mixing and to population transfer by satellite excitation are presented, and the relevance of DQ saturation to the recently developed double-resonance interferometric spectroscopy is discussed.