Design and performance of an oversized-sample 35 GHz EPR resonator with an elevated Q value

Abstract. Continuous wave EPR spectroscopy at 35 GHz is an essential cornerstone in multi-frequency EPR studies and crucial for differentiating multiple species in complex systems due to the improved g tensor resolution compared to lower microwave frequencies. Especially for unstable and highly sensitive paramagnetic centers the reliability of the measurements can be improved by the use of a single sample for EPR experiments at all frequencies. Besides the advantages, the lack of common availability of oversized-sample resonators at 35 GHz often limits scientists to lower frequencies or smaller sample geometries, the latter may be non-trivial for sensitive materials. In this work, we present the design and performance of an oversized-sample 35 GHz EPR resonator with a high loaded Q value up to 3300 well suited for continuous wave EPR and single microwave frequency experiments with low excitation power. The design is driven by electromagnetic field simulations and the microwave characteristics of manufactured prototypes were found in agreement with the predictions. The resonator is based on a cylindrical cavity with a TE₀₁₁ mode allowing for 3 mm sample access. Design targets met comprise high sensitivity, robustness, ease of manufacturing and maintenance. The resonator is compatible with commercial EPR spectrometers and cryostats, allowing for measurements at temperatures down to at least 4 K. To highlight the general applicability, the resonator was tested on metal centers as well as on organic radicals featuring extremely narrow lines.