5 A Portable 14GHz Dual-Mode Pulse and Continuous-Wave Electron Paramagnetic Resonance Spectrometer Using a Subharmonic Direct Conversion Receiver

Electron paramagnetic resonance (EPR) is a powerful spectroscopic technique for direct detection and characterization of free radicals, with high sensitivity and specificity suited for a wide range of uses, including biomedical applications such as site-directed spin labeling and potential applications in quantum technology such as spin qubit control. Unpaired electrons within a static magnetic field B 0 undergo splitting in energy levels (Zeeman effect), and energy transitions may be excited by applying an oscillating field B 1 with a frequency ₀ determined by B 0. In continuous-wave (CW) experiments, B 0 or ₀ is swept, typically with frequency or field modulation to enhance sensitivity. In pulse experiments, B 1 pulses are applied to manipulate the electron spins, and the transient evolution of the magnetization is detected. These methods are complementary: CW EPR offers greater sensitivity while pulse EPR enables more detailed characterization of the coherence and relaxation of the spin system.