A Frequency-Swept, Longitudinal Detection EPR System for Measuring Short Electron Spin Relaxation Times at Ultra-Low Fields.

A frequency-swept longitudinal detection (LOD) EPR system is described for ultra-low field spectroscopy and relaxometry. With the capability of performing simultaneous transmit and receive with -80 dB isolation, this LOD-EPR can capture signals with decay constants in the nanosecond range and in theory even sub-nanosecond range, at fields close to the earth's magnetic field. The theoretical principles underlying this LOD-EPR are based on a fictitious field that accounts for the Z-axis magnetization polarized by a radiofrequency field alone. The electron spin relaxation time is obtained directly from a previously derived equation that describes the relationship between the relaxation time and the spectral peak position. Herein, the first frequency-swept LOD-EPR system is described in detail, along with experimental measurements of the short relaxation time (similar to 30 ns) of the free radical, 2,2-diphenyl-1-picrylhydrazyl, at zero to low field. (C) 2022 Elsevier Inc. All rights reserved.