Analysis of saturation transfer electron paramagnetic resonance spectra in terms of amplitude and phase

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Abstract
A systematic analysis of saturation transfer (ST) electron paramagnetic resonance spectroscopy was developed to separate an observed absorption signal either into an amplitude and a phase spectrum or into an in‐phase and an ST spectrum. We established a phase reference procedure, replacing the low‐power phase null method by a high‐power phase maximum method. This is an extension of the finding that a peak‐to‐peak height and a single integration of the second harmonic absorption display vary sinusoidally as functions of the modulation phase [Y. Shimoyama and H. Watari, Appl. Spectrosc. 3 9, 170 (1985)]. Based on the sinusoidal variation in the peak‐to‐peak height, we found a phase shift at each magnetic field and could evaluate the amplitude and phase of a signal vector. The sinusoidal variation of the single integrated value allowed the absorption signal to be separated into an in‐phase and an ST spectrum. The high‐power phase maximum method provides a remedy of the phase setting problem in ST spectroscopy by which the in‐phase signal can be detected at any microwave field.