LOW-ENERGY EXCITONS IN SOLID C-60

Both the exceptional high symmetry of the C-60 molecule and the molecu lar nature of the solid determine the conditions for a dipole forbidde n lowest excited electronic manifold which extends from 1.8eV to about 3eV. The one-photon activity in this spectral range is due to the Her zberg-Teller vibronic coupling between these forbidden states and high er T-1u excitations. The location of the lowest excited singlet state is performed by two non-linear spectroscopic techniques. First we repo rt on the third harmonic generation spectrum of C-60 thin film. Fittin g of the relatively large number of experimental points with a three-l evel model suggests the presence of a low lying forbidden state at 1.8 7 eV. Preliminary data of two-photon excitation spectroscopy of C-60 s ingle crystal confirm this finding and allows to locate the level at 1 .846 eV. This is in complete agreement with the assignment to a T-1g e xcited state based on the photoluminescence experiments presented by G uss et al. In this issue.