Radical pair recombination reactions are known to be sensitive to the application of both low and high magnetic fields. The application of a weak magnetic field reduces the singlet yield of a singlet-born radical pair, whereas the application of a strong magnetic field increases the singlet yield. The high field effect arises from energy conservation: when the magnetic field is stronger than the sum of the hyperfine fields in the two radicals, S → T_± transitions become energetically forbidden, thereby reducing the number of pathways for singlet to triplet interconversion. The low field effect arises from symmetry breaking: the application of a weak magnetic field lifts degeneracies among the zero field eigenstates and increases the number of pathways for singlet to triplet interconversion. However, the details of this effect are more subtle and have not previously been properly explained. Here we present a complete analysis of the low field effect in a radical pair containing a single proton and in a radical pair in which one of the radicals contains a large number of hyperfine-coupled nuclear spins. We find that the new transitions that occur when the field is switched on are between S and T₀ in both cases, and not between S and T_± as has previously been claimed. We then illustrate this result by using it in conjunction with semiclassical spin dynamics simulations to account for the observation of a biphasic-triphasic-biphasic transition with increasing magnetic field strength in the magnetic field effect on the time-dependent survival probability of a photoexcited carotenoid-porphyrin-fullerene radical pair.

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关于自由基对反应中的低磁场效应

已知自由基对重组反应对低磁场和高磁场都敏感。施加弱磁场会降低单重态自由基对的单重态产率，而施加强磁场会提高单重态自由基对的单重态产率。高场效应源于能量守恒：当磁场强于两个自由基的超精细场之和时，S→T _±过渡被严格禁止，从而减少了单重态到三重态互变的途径数量。低场效应是由对称性破坏引起的：施加弱磁场会提高零场本征态之间的简并性，并增加单重态到三重态互变的途径数量。但是，这种效果的细节更加微妙，并且以前没有得到适当的解释。在这里，我们对包含单个质子的自由基对和其中一个自由基包含大量超精细偶联核自旋的自由基对进行低场效应的完整分析。我们发现，在两种情况下，打开磁场时发生的新过渡都在S和T ₀之间，而不是在S和T _±之间如先前所声称的。然后，我们将其与半经典自旋动力学仿真结合使用来说明该结果，以说明随着磁场强度对光激发类胡萝卜素的时间依赖性生存概率的影响，随着磁场强度的增加，双相-三相-双相转变的观察结果卟啉-富勒烯自由基对。