Optical-absorption spectra in fullerenes C60 and C70: Effects of Coulomb interactions, lattice fluctuations, and anisotropy.

Abstract

Effects of Coulomb interactions and lattice fluctuations in the optical-absorption spectra of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ are theoretically investigated by using a tight-binding model with long-range Coulomb interaction and bond disorder. Anisotropy effects in ${\mathrm{C}}_{70}$ are also considered. Optical spectra are calculated by using the Hartree-Fock approximation followed by the configuration interaction method. The main conclusions are as follows. (1) The broad peaks at excitation energies, 3.7, 4.7, and 5.7 eV, observed in experiments of ${\mathrm{C}}_{60}$ molecules in a solution, are reasonably described by the present theory. Peak positions and relative oscillator strengths are in overall agreement with the experiments. The broadening of peaks by lattice fluctuations is well simulated by the bond disorder model. (2) The optical gap of ${\mathrm{C}}_{70}$ is larger when the electric field of light is parallel to the long axis of the molecule. The shape of the frequency dispersion also depends on the orientation of the molecule. These properties are common in the free-electron model and the model with Coulomb interactions. (3) The spectrum of ${\mathrm{C}}_{70}$ averaged over bond disorder and random orientations is compared with experiments in a solution. There is an overall agreement about the spectral shape. Differences in the spectra of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ are discussed in connection with the associated symmetry reduction.