Saturation recovery electron paramagnetic resonance spectrometer

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Abstract
A versatile saturation recovery accessory based on a small, special‐purpose timing controller and an efficient mix of coaxial and waveguide microwave components has been added to a commercial electron paramagnetic resonance (EPR) spectrometer. The spectrometer was designed for study of the spin lattice relaxation time of transition metals and free radicals over a wide temperature range, such as are of interest in materials science and metallobiochemistry. A microprocessor‐based timing system was designed to provide a wide dynamic range with a simple user interface. Waveguide phase shifters and rotary vane attenuators were used for their precision and resetability, while coaxial components were used where their superior performance could be exploited. Sensitivity is provided by a low‐noise GaAs field‐effect transistor (FET) microwave preamplifier and a double‐balanced mixer (DBM) detector. A biphase modulator (±180° phase shifter) on the lo side of the DBM provided efficient addition/subtraction of successive recovery curves, obviating slow data transfer to a computer prior to the arithmetic operations. Because the system is designed for the study of inherently broad spectra, it does not use the magnetic field step modulation of some prior spectrometers. In addition, although most EPR spectrometers have been designed with a phase shifter in the reference arm, we have found it more convenient to keep the reference arm phase constant and adjust the phases of the pump and observe arms of this three‐arm bridge spectrometer. The saturation recovery system described here could be implemented on any commercial EPR spectrometer with only minor modifications.