ABSORPTION AT THE DIPOLE-FORBIDDEN OPTICAL GAP OF CRYSTALLINE C-60

We report detailed optical studies of crystalline C60 applying several optical spectroscopic techniques such as absorption, photoluminescenc e, and one-photon and two-photon photoluminescence excitation spectros copy. The energetically lowest purely electronic 0'-0 transition is fo und at 1.846 eV. At low temperatures, the absorption spectrum systemat ically exhibits doublet features with the same energetic splitting as observed for the low-temperature fluorescence spectrum. These transiti on pairs either represent two distinct vibration-induced transition or only one vibration-induced transition and the splitting is then due t o the two distinct alignments of neighbouring C60 molecules (merohedra l disorder). Temperature-dependent absorption measurements show that t he two molecular alignments might indeed be responsible for the double resonances.