High-Field Pulsed ENDOR with Intra-cavity Radiofrequency Coil
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Applied Magnetic Resonance
ORIGINAL PAPER
High‑Field Pulsed ENDOR with Intra‑cavity Radiofrequency Coil G. Annino1 · H. Moons2 · M. Fittipaldi3 · S. Van Doorslaer4 · E. Goovaerts2 Received: 13 July 2020 / Revised: 11 September 2020 / Accepted: 15 September 2020 © The Author(s) 2020
Abstract This study compares the performance of two coil configurations for W-band pulsed ENDOR using a setup with both a radiofrequency ‘hairpin’ coil internal to a microwave non-radiative resonator and Helmholtz-like coils external to the resonator. Evaluation of the different coil performances is achieved via the ENDOR study of two model systems. The efficiencies of the coil configurations are first investigated numerically, showing that a higher radiofrequency current-to-magnetic field conversion factor can be achieved with the intra-cavity coil, with a similar radiofrequency magnetic field uniformity. This result is then confirmed by the broadband ENDOR spectra acquired with the two coil arrangements. A gain in the signal-to-noise ratio enabled by the internal coil of about a factor 10 was observed. In some cases, the high conversion factor of the intra-cavity coil led to a saturation of the ENDOR transitions. The possibility to implement a similar intra-cavity radiofrequency coil configuration in higher field spectrometers is finally discussed.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s0072 3-020-01269-z) contains supplementary material, which is available to authorized users. * G. Annino [email protected] 1
Istituto per i Processi Chimico-Fisici, IPCF-CNR, via G. Moruzzi 1, 56124 Pisa, Italy
2
ECM Laboratory, Department of Physics, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium
3
Department of Physics and Astronomy and INSTM Research Unit, University of Florence, via G. Sansone 1, 50019 Sesto Fiorentino, Italy
4
BIMEF Laboratory, Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium
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1 Introduction Among the various techniques developed in the field of Electron Paramagnetic Resonances (EPR), the Electron-Nuclear Double Resonance (ENDOR) spectroscopy owes its popularity to the ability to provide detailed information on hyperfine interactions and on the electronic structure of paramagnetic centers. Initially proposed in 1956 by Feher as a transient technique [1–3], it mainly established itself in its continuous-wave (cw) version during the first decades [4, 5]. Pulsed ENDOR techniques, made appealing by potentially stronger as well as less distorted signals [6], became important several years after their initial proposal [7, 8], thanks to the development of the necessary electronics. Nowadays, cw and pulsed ENDOR are both largely utilized in EPR laboratories. cw ENDOR requires the right balance between the longitudinal relaxation time of the electron spins and that of the nuclear spins. Usually, it provides a detectable signal only in a limited temperature r
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