Quantum beats of a multiexciton state in rubrene single crystals

Abstract

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.