C. Taliani et al., LOW-ENERGY EXCITONS IN SOLID C-60, Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals, 256, 1994, pp. 187-198
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.