Fay Thomas P, Manolopoulos David E
Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Chem Phys. 2019 Apr 21;150(15):151102. doi: 10.1063/1.5095204.
Magnetic field effects on radical pair reactions arise due to the interplay of coherent electron spin dynamics and spin relaxation effects, a rigorous treatment of which requires the solution of the Liouville-von Neumann equation. However, it is often found that simple incoherent kinetic models of the radical pair singlet-triplet intersystem crossing provide an acceptable description of experimental measurements. In this paper, we outline the theoretical basis for this incoherent kinetic description, elucidating its connection to exact quantum mechanics. We show, in particular, how the finite lifetime of the radical pair spin states, as well as any additional spin-state dephasing, leads to incoherent intersystem crossing. We arrive at simple expressions for the radical pair spin state interconversion rates to which the functional form proposed recently by Steiner et al. [J. Phys. Chem. C 122, 11701 (2018)] can be regarded as an approximation. We also test the kinetic master equation against exact quantum dynamical simulations for a model radical pair and for a series of PTZ-Ph-PDI molecular wires.
磁场对自由基对反应的影响源于相干电子自旋动力学和自旋弛豫效应的相互作用，对其进行严格处理需要求解刘维尔 - 冯·诺依曼方程。然而，人们常常发现，自由基对单重态 - 三重态系间窜越的简单非相干动力学模型能够对实验测量结果给出可接受的描述。在本文中，我们概述了这种非相干动力学描述的理论基础，阐明了它与精确量子力学的联系。我们特别展示了自由基对自旋态的有限寿命以及任何额外的自旋态退相如何导致非相干系间窜越。我们得出了自由基对自旋态相互转换速率的简单表达式，斯坦纳等人[《物理化学杂志C》122, 11701 (2018)]最近提出的函数形式可被视为对该表达式的一种近似。我们还针对一个模型自由基对以及一系列PTZ - Ph - PDI分子线，将动力学主方程与精确量子动力学模拟进行了对比检验。
