Self-trapped polaron exciton in neutral fullereneC60

The luminescence of ${\mathrm{C}}_{60}$ was investigated as a function of temperature between 300 and 10 K and for various oxygen concentrations. Its intensity was found to increase strongly with decreasing temperature down to about ${\mathit{T}}_1$=100 K and to decrease again for a further reduction of T. Below 100 K, new structures appeared in the spectra. The experiments are described by a self-localized polaron exciton and by a diffusing free exciton above and below ${\mathit{T}}_1$, respectively. The important difference between the two regions is the free rotation of the ${\mathrm{C}}_{60}$ above and the definite orientation below ${\mathit{T}}_1$.