Double-quantum coherent transients in a two-level spin system: a vectorial model

The coherent transient regimes of the double-quantum (DQ) resonances are theoretically investigated in a S = 1 2 electron spin system. A rotating reference frame is found in which the time evolution of the spin system during the DQ transient regimes may be described by a set of Bloch-like equations. It is shown that the spin precessional motions manifest themselves in the laboratory reference frame as an amplitude modulation of the second harmonic components of the transverse magnetization. A vectorial model is used in particular to calculate the dependence of the nutational frequency of the DQ transitions on the intensity, the polarization and the detuning of the input radiation. The results are compared with recently reported observations.