Localized and extended exciton states in solid C-60 studied by photoluminescence at high pressure

We have measured the photoluminescence spectra of C-60 Single crystals at T approximate to 10 K and pressures up to 4.0 GPa. The spectrum at normal pr essure consists of a number of peaks, previously related to emission of loc alized Frenkel excitons. The pressure dependence of the spectrum shows a gr adual red shift and a considerable redistribution of peak intensities. The intensities of almost all peaks decrease with increasing pressure, whereas one of them rapidly grows and dominates at P approximate to 1.7 GPa. The pr essure-induced shift of this peak is considerably higher than that of other peaks, and the pressure dependence of its position is strongly related to the pressure dependence of the direct gap. This peak may be associated with the vibronically assisted transition from the T-1(1g) extended exciton sta te.