Theoretical study of molecular electronic excitations and optical transitions of C60

TL;DR

This study uses ab initio configuration interaction calculations to analyze the excited states and optical transitions of C60, revealing higher energy positions for key optical lines than previously estimated, with minor effects from double excitations.

Contribution

It provides a detailed ab initio analysis of C60's excited states, correcting earlier semiempirical estimates and clarifying the nature of optical transitions.

Findings

Two main optical lines are at ~5.8 eV and ~6.3 eV.

Lowest excitation is at ~2.5 eV.

Double excitations cause minor corrections.

Abstract

We report results on ab initio calculations of excited states of the fullerene molecule by using configuration interaction (CI) approach with singly excited determinants (SCI). We have used both the experimental geometry and the one optimized by the density functional method and worked with basis sets at the cc-pVTZ and aug-cc-pVTZ level. Contrary to the early SCI semiempirical calculations, we find that two lowest $^1 T_{1u} \leftarrow {}^1 A_g$ electron optical lines are situated at relatively high energies of ~5.8 eV (214 nm) and ~6.3 eV (197 nm). These two lines originate from two $^1 T_{1u} \leftarrow {}^1 A_g$ transitions: from HOMO to (LUMO+1) ($6h_u \to 3t_{1g}$) and from (HOMO--1) to LUMO ($10h_g \to 7t_{1u}$). The lowest molecular excitation, which is the $1 ^3 T_{2g}$ level, is found at ~2.5 eV. Inclusion of doubly excited determinants (SDCI) leads only to minor corrections to this picture. We discuss possible assignment of absorption bands at energies smaller than 5.8 eV (or $\lambda$ larger than 214 nm).