Electronic excited states in C60 thin film as revealed by spectroscopic measurements: luminescence excitation spectra and temperature dependence

Spectroscopic properties of C60 thin film have been studied by the measurement of luminescence excitation spectra and the temperature dependence of the absorption spectrum and luminescence intensity. Unlike isolated molecules in solution, the luminescence excitation spectrum of a thin film showed a considerably different structures than its absorption spectrum. Excitation at shorter wavelength appears to give a lower quantum efficiency, indicating an excitation energy dependent branching in the relaxation process of excited states in solid C60. The increase in luminescence intensity when temperature is lowered is well described by a thermally activated quenching with an activation energy of 0.12eV. A change in the absorption spectrum of C60 thin film around the phase transition temperature suggests the effect of symmetry breaking on the oscillator strength of the forbidden transition.