Cross-polarisation edited ENDOR

Abstract

Electron-nuclear double resonance (ENDOR) is a fundamental technique in electron paramagnetic resonance (EPR) spectroscopy that directly detects hyperfine transitions of nuclei coupled to a paramagnetic centre. Despite its wide use, spin-sensitivity and restricted spectral resolution in powder samples pose limitations of this technique in modern application fields of EPR. In this contribution, we examine the performance of an ENDOR pulse sequence that utilises a preparation scheme different from conventional Davies ENDOR. The scheme is based on electron-nuclear cross-polarisation (eNCP), which requires concomitant microwave (MW) and radio-frequency (RF) irradiation satisfying specific matching conditions between the MW and RF offsets and the hyperfine coupling. Changes in nuclear polarisation generated during eNCP can be detected via a conventional ENDOR read-out sequence consisting of an RF π-pulse followed by EPR-spin echo detection. Using 1H-BDPA as a standard sample, we first examine the CP matching conditions by monitoring the depolarisation of the electron spin magnetisation. Subsequently, so-called CP-edited ENDOR spectra for different matching conditions are reported and analysed based on the provided theoretical description of the time evolution of the spin density matrix during the experiment. The results demonstrate that CP-edited ENDOR provides additional information with respect to the sign of the hyperfine couplings. Furthermore, the sequence is less sensitive to nuclear saturation effects encountered in conventional ENDOR.