LATTICE DISTORTION AND ENERGY-LEVEL STRUCTURES IN DOPED C-60 AND C-70 MOLECULES STUDIED WITH THE EXTENDED SU-SCHRIEFFER-HEEGER MODEL - POLARON EXCITATIONS AND OPTICAL-ABSORPTION

We extend the Su-Schrieffer-Heeger model of polyacetylene to the cases of C60 and C70 molecules. The results of our numerical calculations of the undoped systems agree well with the known results. When the system (C60 or C70) is doped with one or two electrons (or holes), additional charges accumulate nearly along an equatorial line of the molecule. The dimerization becomes weaker along the same line. Two energy levels intrude deeply into the gap; the intrusion is deeper in C70 than in C60. Therefore, "polarons" are predicted in doped fullerenes. We calculate the optical-absoprtion coefficient for the C60 fullerene in order to consider how the polarons will be observed. It is predicted that an additional absorption peak appears at an energy lower than the intergap transition peaks of the undoped C60. It is found that the C60 and C70 fullerenes are closely related in their electronic structures as well as lattice geometries.