Response to Double Irradiation of a Nuclear Spin System: General Treatment and New Multiple Quantum Transitions

A general theory has been developed to calculate the response, in NMR conditions, of a spin system irradiated by two nearly resonant waves having different frequencies and intense enough to produce an appreciable saturation. The absorption signal has been calculated employing the statistical operator technique, treating the electromagnetic field by the second quantization formalism. Two kinds of transitions are expected: The first one arises from processes in which $n$ photons of one wave are emitted and $n$ photons of the other wave are absorbed, the second one arises from processes involving the emission of $n$ photons of one wave and the absorption of $\text{n\hspace{0.17em}±\hspace{0.17em}1}$ photons of the other. In the first case all the transitions coalesce in a single dispersionlike signal, which occurs when the frequencies of the two waves are nearly the same; in the second case a many‐line spectrum occurs in which emission and absorption lines are present. The experimental results here reported strictly agree with theoretical previsions.