Mims Electron-Nuclear Double Resonance (ENDOR) with chirp microwave pulses

We present a modification of the Mims Electron-Nuclear DOuble Resonance (ENDOR) experiment that uses microwave (MW) chirp pulses to enable broadband excitation in W-band (94GHz) Electron Paramagnetic Resonance (EPR) spectroscopy and Fourier transform (FT) of the resulting stimulated echoes. The method is demonstrated on a nitroxide-¹⁹F model system and yields two-dimensional spectra that correlate hyperfine couplings with the EPR spectrum. In contrast to standard, time consuming and costly orientation-selective ENDOR measurements, this approach requires just one experiment at a single EPR resonance field position. At the same time, the resolution in the EPR dimension is increased. This provides additional information on the orientation of the hyperfine coupling tensor and enables the direct observation of chemical shielding anisotropy at W-band frequency. We analyze the effect of chirp pulses on the Mims blind spot function using CHirp Echo Epr SpectroscopY (CHEESY). Additionally, we illustrate that FT of spin echoes can be beneficial even without the availability of chirp pulses, using high-power rectangular MW pulses at Q-band (34GHz). Overall, our results suggest that the combination of Mims ENDOR spectroscopy with FT of MW echoes can be a valuable method to obtain detailed information on electron-nuclear spin interaction tensors and will be potentially useful to disentangle complex, multi-component ENDOR spectra.