Sensitivity Enhancement in Pulsed Hyperfine EPR Spectroscopy with Hadamard-Encoded Acquisition

Electron-nuclear double resonance (ENDOR) is a powerful tool for determining the spatial and electronic structure of paramagnetic systems. It often suffers from a limited signal-to-noise ratio (SNR), particularly for small hyperfine couplings, which correspond to long-range electron-nuclear distances. We implement a Hadamard frequency multiplexing strategy to enhance the sensitivity in frequency-domain ENDOR spectroscopy. This method makes use of simultaneous or sequential excitation of multiple nuclear frequencies within a single pulse sequence, with spectral reconstruction via a Hadamard transform. Using fluorine ENDOR of fluorinated small molecules and spin-labeled proteins, we demonstrate up to 2-fold improvement in SNR. Approaches that mitigate the limitations of radiofrequency power and relaxation effects are presented, making this strategy useful for both organic radicals and paramagnetic metal complexes. Beyond ENDOR, Hadamard encoded acquisition also improves electron double resonance detected NMR sensitivity, suggesting broad applicability across EPR methods that rely on frequency sweeps.