THEORY OF NONLINEAR-OPTICAL RESPONSE OF EXCITONS IN SOLID C-60

We present a theoretical study of the dispersion of Frenkel-type excit ons in the low-temperature Pa(3) over bar phase of solid C-60. Exciton propagation is accomplished by an exchangelike two-step process, wher e an electron-hole pair on a given C-60 molecule is split by the inter molecular kinetic energy and recombines on a neighboring molecule. We apply our results to the theoretical calculation of the nonlinear opti cal susceptibility for second-harmonic generation, for which we find r easonable agreement with experiment.