Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570, Japan.
J Chem Phys. 2019 Dec 7;151(21):214501. doi: 10.1063/1.5127904.
Using the density operator formalism, a simple analytical model is developed to study low magnetic field effects on triplet pair annihilations in organic solids. Analysis is restricted to canonical orientations where two identical triplet molecules have the same orientation and the direction of the external magnetic field is parallel to one of the principle axes of the dipolar coupling tensor for a triplet. The analytical solution reveals that the low magnetic field effect in the triplet pair arises from the anisotropic dipole-dipole coupling in a triplet. In the presence of the dipole-dipole coupling, the spin quantization axis for each triplet gradually changes with the increase of the external magnetic field from zero field to high field. The low magnetic field effect reaches a maximum when the Zeeman splitting between the spin states matches a dipole-dipole coupling component orthogonal to the external magnetic field direction. The result is also discussed with the low magnetic field effect in the radical pair with one isotropic hyperfine coupling.
利用密度算符形式，建立了一个简单的分析模型，用于研究低磁场对有机固体中三重态对湮没的影响。分析仅限于正则取向，其中两个相同的三重态分子具有相同的取向，并且外磁场的方向与偶极耦合张量的一个主轴平行对于三重态。分析解表明，三重态对中的低磁场效应源于三重态中的各向异性偶极-偶极耦合。在外磁场存在的情况下，每个三重态的自旋量子化轴随着外磁场从 0 场到高场的增加而逐渐变化。当自旋态之间的塞曼分裂与垂直于外磁场方向的偶极-偶极耦合分量匹配时，低磁场效应达到最大值。还讨论了具有各向同性超精细耦合的自由基对中的低磁场效应。
