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Excited States as a Probe of Structural Phase Transitions in Polyphenyl Crystals

Phase transitions in biphenyl and p-terphenyl crystals were studied by the laser induced fluorescence and the absorption spectroscopy. In biphenyl, besides the intrinsic band starting from a false zero-phonon peak at 302.5nm, an excimer-like broad band appeared at about 480nm. The fluorescence decay curve changes near T c =17K and 40K, since the fluctuations of tortional vibration near T c have an influence on the energy transfer rate from the intrinsic excited state to the excimer-like state. The deviation from the Urbach-rule below T 1 = 40K in the absorption tail of biphenyl is also attributed to the tortional amplitude mode. In p-terphenyl, unlike the biphenyl case, sharp absorption peaks to the tortional levels in the excited state appears up to 50K and the fluorescence intensities gradually change with temperature in a characteristic way.