We observe quantum beats in the nanosecond-scale photoluminescence decay of rubrene single crystals after photoexcitation with short laser pulses in a magnetic field in the range of 0.1 to 0.3 T. The relative amplitude of the quantum beats is of the order of 5%. Their frequency is 1.3 GHz when the magnetic field is oriented parallel to the twofold rotation axis of the rubrene molecules and decreases to 0.6 GHz when the magnetic field is rotated to the crystal's molecular stacking direction. The amplitude of the quantum beats decays alongside the non-oscillatory photoluminescence background, which at low excitation densities has an exponential decay time of 4.0 ± 0.2 ns. We interpret this as the effective lifetime of a multiexciton state that originates from singlet-fission and can undergo geminate recombination back to the singlet state.

中文翻译：

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红荧烯单晶中多激子态的量子节拍

我们在 0.1 到 0.3 T 的磁场中用短激光脉冲光激发后观察到红荧烯单晶的纳秒级光致发光衰减中的量子节拍。量子节拍的相对幅度约为 5%。当磁场平行于红荧烯分子的双重旋转轴取向时，它们的频率为 1.3 GHz，当磁场旋转到晶体的分子堆叠方向时，它们的频率降低到 0.6 GHz。量子节拍的振幅与非振荡光致发光背景一起衰减，在低激发密度下，其指数衰减时间为 4.0 ± 0.2 ns。我们将其解释为源自单线态裂变的多激子态的有效寿命，并且可以经历成对重组回到单线态。