Quantum beats of a multiexciton state in crystalline rubrene

We have observed quantum beats in the photoluminescence dynamics of the organic molecular crystal rubrene following photoexcitation by a femtosecond pulsed laser. The application of a magnetic field up to 0.3 T, created by a neodymium permanent magnet, produces periodic modulations in the photoluminescence dynamics. The beat frequency ranges between 0.6 GHz to 1.3 GHz and is dependent upon the relative orientation of the magnetic field and the crystalline axes. The amplitude of the beats increases with increasing magnetic field strength, peaking at about 5% of the non-oscillatory background. These beats are indicative of a multiexciton state consisting of a spin-coherent pair of triplet excitons existing during the singlet-to-triplet exciton fission conversion process.