Theory of nonlinear optical response of excitons in solidC60

We present a theoretical study of the dispersion of Frenkel-type excitons in the low-temperature Pa3¯ phase of solid ${\mathrm{C}}_{60}$. Exciton propagation is accomplished by an exchangelike two-step process, where an electron-hole pair on a given ${\mathrm{C}}_{60}$ molecule is split by the intermolecular kinetic energy and recombines on a neighboring molecule. We apply our results to the theoretical calculation of the nonlinear optical susceptibility for second-harmonic generation, for which we find reasonable agreement with experiment. © 1996 The American Physical Society.