Laser-induced fluorescence spectra and phase transitions in biphenyl crystals

Fluorescence spectra of biphenyl single crystals grown from methyl-alcohol solution were studied over a wide temperature range by the 295 nm UV light from a dye laser system. A fluorescence band near 530 nm originating from photo-chemically produced impurities was found to be composed of a zero phonon line and a phonon side band manifesting some of the low frequency lattice vibrations of the host crystal and the local modes of the impurity centers as well. We found that the 186 cm(-1) lattice mode is coupled with the excited state of the impurity and the temperature dependence of its intensity reflects the orderness of the structural phase transitions of the host crystal at 40 K and 17 K.