Origin of intrinsic luminescence from C60 single crystals

Optical properties of pristine C-60 single crystals have been studied in the temperature range from 4 K to 300 K by applying several spectroscopic techniques such as absorption, photoluminescence under one-photon and two-photon excitation, and site-selective luminescence line narrowing spectroscopy. Localized absorption characterized by the prominent vibrational structures is found around 1.69 eV below the fundamental absorption edge (1.81eV), which apparently exhibits mirror-images of the well-known spectrum of photoluminescence called type A. Upon selective excitation into the band, luminescence line narrowing is observed: The spectra consist of sharp vibrational lines attributed to the progressions of eight Hg and two Ag modes and libron sidebands. These results indicate that the type A luminescence originates from the inhomogeneously broadened localized state which may be stabilized by the Symmetry lowering and local rotational disorder spread over more than one molecule.