Impact of high-dielectric-loss materials on the microwave field in EPR experiments

A dielectric material distorts the microwave held inside an EPR resonator, which results in distortion of the EPR signal from spins inside the material. In this paper, the effects of a spherical bulb filled with a dielectric liquid such as water dr a water-ethanol mixture were examined. EPR spectra were recorded for small samples inside and outside of the sphere. The studies include CW and ESE experiments at two microwave frequencies, X band (9.2 GHz) and L band (1.03 GHz). The double integral (area) of an EPR signal depends on B-1(2) at the position of the sample, causing a large difference in EPR signal intensities between samples in regions of different dielectrics. The phase of the EPR signal also is affected by the presence of the dielectric. These results were compared with three methods of calculating electromagnetic fields (quasi-static method, plane-wave-superposition method, and numerical analysis). Good agreement was found between experimental and calculated results. (C) 1996 Academic Press, Inc.