Exciton and lattice-fluctuation effects in optical spectra of C60

TL;DR

This paper develops a theoretical model to analyze optical excitations in C60, incorporating Coulomb interactions and lattice fluctuations, revealing the nature of excitons and the effects of disorder on optical transitions.

Contribution

It introduces a combined tight binding and Coulomb interaction model with lattice disorder for C60, providing insights into exciton character and transition mechanisms.

Findings

Photo-excited states are mainly Frenkel excitons with small charge-transfer components.

Lattice fluctuations can activate dipole-forbidden transitions in C60.

Appropriate Coulomb interaction strength matches experimental optical spectra.

Abstract

A theory of optical excitations by using a tight binding model with long-range Coulomb interactions is developed. The model is applied to a C60 molecule and a cluster, and is treated by the Hartree-Fock approximation followed by a configuration interaction method. Lattice fluctuations are taken into account by a bond disorder model. We first examine what strength of Coulomb interaction is appropriate to describe the electronic structures observed by photo-electron and optical absorption spectroscopy. Then, we demonstrate that the photo-excited states are mainly intramolecular (i.e. Frenkel) excitons with small charge-transfer components. We examine to what extent the dipole forbidden transitions of a single C60 molecule become dipole-allowed by lattice fluctuations or intermolecular interactions.