Second-Harmonic Generation and Spin Decoupling in Resonant Two-Level Spin Systems

A paramagnetic spin system is intrinsically nonlinear, and especially so at resonance; therefore, in the presence of a strong rf magnetic field it exhibits magnetic polarization at harmonic frequencies. We report here a complete set of experimental data on second-harmonic generation by a resonant two-level spin system. A theory is also developed, using the density-matrix formalism, which fully accounts for the experimental data. It is suggested that these harmonic signals are convenient to use for the measurement of the transverse relaxation time in saturated and unsaturated spin systems.