Direct determination of single-crystal parameters for axial paramagnetic resonance centers in polycrystalline-amorphous substances

Abstract

The paramagnetic resonance powder spectrum of an electron or nuclear spin system involving an isotropic Zeeman and an axially symmetric, first-order fine-structure splitting has been numerically investigated for the purpose of finding a simple, direct, and accurate method for determining its single-crystal parameters in polycrystalline-amorphous samples. It is shown that from the positions and relative intensities of some selected first- and second-derivative peaks of the axial fine-structure powder spectrum, one can directly determine with high accuracy the associated coupling constant, single-crystal line shape, and single-crystal linewidth for the external magnetic field perpendicular as well as parallel to the axially symmetric axis. The results of an experimental application of this method to the case of a randomly oriented zinc chelate biradical system are presented. The new powder spectrum method is applicable to first-order axially symmetric ESR and NMR quadrupole and dipole-dipole pair centers with isotropic g values under rigid state or slow tumbling orientational motion in polycrystalline-amorphous substances.