Optical characterization of excited states in films of undoped C 60

Abstract

We have used various optical characterization techniques to study the electronic excited states in icosahedral clusters of C 60 thin films produced by high-vacuum evaporation onto sapphire substrates. These techniques include c.w. photoluminescence (PL), photomodulation (PM) and spin-dependent PL and PM. We have found a modestly strong PL band at 1.72 eV, with two phonon replicas 185 meV apart. This phonon is the most strongly coupled C 60 pentagonal pinch mode and therefore shows that the PL band is intrinsic to C 60. From its flat modulation frequency response up to 100 kHz, we conclude that the PL lifetime is shorter than 3 μs, implying that it is due to singlet exciton recombination. The PM spectrum contains a single band peaking at about 1 eV, which is correlated with a derivative-like feature around 2.3 eV; no photoinduced IR-active vibrations have been discovered. From the additional study of the frequency and excitation dependencies, we conjecture that the PM is due to charged carriers which recombine with bimolecular kinetics.