Fig. 4: Enhanced signal decay with applied AC field.

A Integrated signal intensity of decaying component S_d (see Fig. 3B) with changing AC frequency, with θ ≈ 75^∘, and τ = 73 μs. Decays are normalized against their value at 20 ms and truncated at 5 s (corresponding to ≈ 10⁵ pulses). Enhanced decay occurs at resonance condition (dashed line). Solid line is a spline fit guide to the eye. Linewidth is estimated ≈ 223 Hz (marked) from a Gaussian fit. B Average Hamiltonian analysis. Simulated signal, here assuming θ = π/2 and no dipolar coupling between ¹³C nuclei. Resonance is expected at \({f}_{{{{{{{{\rm{res}}}}}}}}}=1/4\tau\). C Phasor representation of toggling frame Hamiltonians for B under an applied (i) DC field and (ii) AC field at resonance. D Resonance frequency scaling. Points show experimentally extracted resonance frequency from a second harmonic intensity (see Fig. S1) for varying pulse widths t_p but fixed t_acq. Solid line is a theoretically predicted \({f}_{{{{{{{{\rm{res}}}}}}}}}\), showing good agreement. E Signal traces for representative points in A (large markers i-iv), corresponding to f_AC = DC, 5 kHz, 2.885 kHz (resonance) and 2.8 kHz (slightly off-resonance). For AC fields near resonance (ii,iii), we observe rapid decays and sharp jumps (see See supplementary online material). Far from resonance (i,iv), the decays exhibit slowly-decaying profiles. F Logarithmic scale plot of the data in E, plotted against \(\sqrt{t}\). Signal decays far from the resonance have a characteristic \(\propto \exp (-{t}^{1/2})\) profile³¹. Decays close to resonance (ii,iii), however, show steeper slopes. G Scaling of resonance linewidth in A as a function of the number of pulses N applied, estimated from a Gaussian fit.