Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches ±16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is ±3.
@article{arxiv.2601.14321,
title = {High-Resolution Electron Paramagnetic Resonance},
author = {Colin J. Stephen and Anton Tcholakov and Maik Icker and Stuart M. Graham and Xiaoming Zhao and Robert Day and Jeanette Chattaway and T. John S. Dennis and Wolfgang Harneit and Gavin W. Morley},
journal= {arXiv preprint arXiv:2601.14321},
year = {2026}
}