The phase behavior of saturation transfer electron spin resonance (ST-ESR) was studied with the use of the second harmonic absorption (V2) spectrum of the spin label (2,2,6,6-tetramethylpiperidine-1-oxyl) in glycerol-water solution. Upon application of an intense microwave field (0.508 G), a pronounced nonzero component was observed at the modulation phase where the peak-to-peak amplitude is expected to be zero. This component is the saturation transfer spectrum which is latent at the phases giving intense V2 signals. A single intergration of these obtained spectra yields a significant integrated value which varies in a sinusoidal manner. The phase discrepancy was found to be 70 degrees between the variations of the peak-to-peak amplitude and the integrated value.
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