Quantum Beats of a Multiexciton State in Rubrene Single Crystals

TL;DR

This study observes quantum beats in rubrene single crystals' photoluminescence, revealing insights into multiexciton states and singlet-fission dynamics influenced by magnetic fields.

Contribution

It reports the first observation of quantum beats in rubrene crystals' photoluminescence related to multiexciton states from singlet-fission.

Findings

Quantum beats occur at GHz frequencies depending on magnetic field orientation.

The effective lifetime of the multiexciton state is approximately 4 ns.

Quantum beat amplitude is about 5% of the total photoluminescence.

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 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 two-fold 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 \pm 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.