REABSORPTION EFFECT ON THE FLUORESCENCE-SPECTRA AND THE LIBRATION OF THE PHENYL RING IN PARA-TERPHENYL CRYSTALS

The fluorescence and the excitation spectra of p-terphenyl crystals have been measured at 10-290 K. At higher temperature, a new band is observed in the lower energy region of the excitation spectrum which is located at about 352 nm at 290 K. This band is induced by the libration of the central phenyl ring in the double well-potential. The existence of its potential barrier is confirmed by the analysis of the temperature dependence of the fluorescence intensity, and the barrier height is about 0.027 eV. As this band overlaps with the fluorescence spectrum, the high energy region of the fluorescence spectrum is eliminated by the reabsorption of the fluorescent photon at higher temperatures.