Absorption at the dipole-forbidden optical gap of crystalline C60

We report detailed optical studies of crystalline C60 applying several optical spectroscopic techniques such as absorption, photoluminescence, and one-photon and two-photon photoluminescence excitation spectroscopy. The energetically lowest purely electronic 0′-0 transition is found at 1.846 eV. At low temperatures, the absorption spectrum systematically exhibits doublet features with the same energetic splitting as observed for the low-temperature fluorescence spectrum. These transition pairs either represent two distinct vibration-induced transitions or only one-vibration-induced transition and the splitting is then due to the two distinct alignments of neighbouring C60 molecules (merohedral disorder). Temperature-dependent absorption measurements show that the two molecular alignments might indeed be responsible for the double resonances.