Temperature dependence of the band gap in C-60 crystals

The temperature dependence (13-300 K) of the optical-allowed direct band gap E(g)(T) deduced from the transmission and photoconductivity spectra in high-quality C-60 crystals is presented. A strong temperature-dependent E(g)(T) is observed and attributed to the electron-phonon interaction via the localized C-60 phonon modes. In addition, a discontinuous change for dE(g)/dT around 249 K is reported due to the orientational order/disorder phase transition. Finally, three discrete localized bands are observed and the photoconduction for these localized bands is discussed in terms of photothermal ionization. (C) 1996 American Institute of Physics.