Single Crystalline Shape Research Articles

Fluorescence‐Detected Low Field Resonances in T–T Annihilation in the A‐TCNB Single Crystal Lineshape and Dynamic Parameters

AbstractAn investigation is made of the line shape of the low field resonances observed in the dependence of the fluorescence of a single A‐TCNB CT crystal on the external magnetic field. It is shown that the resonance is caused by avoided crossing of magnetic sublevels of a single T‐exciton arising when the direction of the field is close to the axis of the fine interaction tensor. An analytic expression for the resonance line shape is obtained. The spin–lattice relaxation rate of T‐excitons, τ−1 = (1.0 ± 0.3) × 107 s−1 as well as the rate of jumps between the planes of fast exciton migrations, ω1 = (5 ± 2) × 107 s−1, are determined.

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

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.