Photoluminescence and near-edge optical absorption in the low-temperature phase of pristine C60 single crystals

Photoluminescence and near-band-edge electronic structure of pristine C-60 single crystals have been investigated in the low temperature phase below 90 K. 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 absorption band are assigned to the vibronic progressions coupled with the Raman-active gerade-symmetry modes of individual C-60 molecules (i.e., eight h(g), two a(g), and their combinations), accompanied by phonon-sidebands due to cooperative librations and intermolecular translations. The origin of photoluminescence and its relaxation 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.