Time-resolved Spectroscopic Studies on the basic Photo-induced Processes in C_<60> Single Crystals

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Photoluminescence and near-band-edge electronic structure of pristine C_<60> single crystals have been investigated in the low temperature phase below 90K. By applying two-photon excitation spectroscopy, a monomorphic spectrum is extracted from the several crystals which exhibit apparent diversity for ordinary one-photon excitation. Also, a localized absorption band with discrete vibronic sub-peaks is clearly confirmed at 1.685 eV below the fundamental absorption edge (1.815 eV). Overall features of the luminescence-line-narrowing spectrum obtained by site-selective excitation into the localized absorp tion band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C60 molecules (i.e., eight h_g, two a_g, and their combinations), accompanied by phonon-side-bands due to cooperative librations and intermolecular translations. The origin of photo luminescence and its relaxtion kinetics are fully discussed in connection with the inhomogeneously broadened localized states which are presumably due to perturbed Frenkel excitons stabilized with symmetry-lowering by adjacent intrinsic defect or some lattice disorder.