RELAXATION PROCESS OF THE SELF-TRAPPING EXCITON IN C60

When ${\mathrm{C}}_{60}$ is photoexcited, a self-trapping exciton (STE) is formed: The bond structure is distorted from symmetry ${\mathit{I}}_{\mathit{h}}$ to ${\mathit{D}}_{5\mathit{d}}$ while the states ${\mathit{A}}_{1\mathit{u}}$ and ${\mathit{A}}_{2\mathit{u}}$ are pulled into the energy gap from highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. A dynamical scheme is employed to simulate the relaxation process of STE. The evolution of both bond structure and electronic states shows that the relaxation time to form STE is about 100 fs. It should be noted that this relaxation time is much shorter than that of charge transfer in ${\mathrm{C}}_{60}$. The origin for this large difference is discussed. \textcopyright{} 1996 The American Physical Society.