Observation of triplet state of charge-transfer excitons in C60 thin film

We have studied formation and energy structure of triplet states of charge-transfer excitons in C60 thin film by means of pump and probe spectroscopy in a vast time domain from 100 fs to 100 ms. In solid C60, absorption bleaching which lasts up to several hundred milliseconds is observed slightly below the absorption band due to charge-transfer excitons (CT excitons). This provides a spectroscopic evidence for existence of an inter-molecular triplet state due to CT excitons in solid C60. The energy splitting between singlet and triplet CT exciton states is 0.09 eV, and the lifetime of triplet CT excitons is ∼50 ms.