Non-linear signal detection improvement by radiation damping in single-pulse NMR spectra

When NMR lines overlap and at least one of them is affected by radiation damping, the resonance line shapes of all lines are no longer Lorentzian. We report the appearance of narrow signal distortions, which resemble hole-burnt spectra. This new experimental phenomenon facilitates the detection of tiny signals hidden below the main resonance. Theoretical analysis based on modified Maxwell-Bloch equations shows that the presence of strong transverse magnetization creates a feedback through the coil, which influences the magnetization of all spins with overlapping resonance lines. In the time domain this leads to cross-precession terms between magnetization densities, which ultimately cause non-linear behavior. Numerical simulations corroborate this interpretation. Positive feedback: The precessing magnetizations of the two species M_a and M_b can influence each other through the feedback field of the detection circuit (radiation damping field, see picture). The resulting spectra exhibit surprising line shapes, such as the appearance of a hole, allowing detection of hidden tiny signals.