Pulsed Electron-Nuclear Double Resonance in the Fourier Regime.

Nuclear magnetic resonance (NMR) spectroscopy provides atomic-level molecular structural information.  However, in molecules containing unpaired electron spins, NMR signals are difficult to measure directly.  In such cases, data is obtained using the electron-nuclear double resonance (ENDOR) method, where nuclei are detected through their interaction with nearby unpaired electron spins.  Unfortunately, electron spins spread the ENDOR signals, which challenges current acquisition techniques, often resulting in low spectral resolution that provides limited structural details.  Here, we show that by using miniature microwave resonators to detect a small number of electron spins, integrated with miniature NMR coils, one can excite and detect a wide bandwidth of ENDOR data in a single pulse.  This facilitates the measurement of ENDOR spectra with narrow lines spread over a large frequency range at much better spectral resolution than conventional approaches, which helps reveal details of the paramagnetic molecules' chemical structure that were not accessible before.