Optical Spectroscopy of the Fullerenes C(60) and C(70)

The fullerenes are a recently discovered third allotrope of carbon. The ability to produce these unusual molecules in gram quantities has led to intensive investigation of their properties. Optical spectroscopies provide powerful tools for investigating the electronic states of materials. We have used the techniques of resonant Raman scattering (RRS), electroabsorption (EA), cw and transient photomodulation (PM) and transient photoinduced resonant Raman scattering (TPRRS) to probe optical excitations and their dynamics in C_{60} and C _{70} thin films. The results of these investigations show strong evidence for charge transfer transitions with energies of 2.4 and 2.15 eV in C_{60} and C_{70} respectively. Charge carriers can be formed mainly by optical excitation above the charge transfer transition. Lower energy excitation results in the formation of singlet Frenkel excitons. These singlet excitons decay primarily nonradiatively; however, there is no evidence of intersystem crossing for times less than 3 ns in either C_{60} or C_{70} films. The triplet excitons in the thin films are formed either by singlet fission or by charge carrier recombination. The presence of charge transfer transitions has implications for conductivity and nonlinear optics in C_ {60} and C_{70} films.