Structural phase transitions of polyphenyl crystals studied by fluorescence and absorption spectroscopy

Structural phase transitions in biphenyl and p-terphenyl crystals are studi ed by fluorescence and absorption measurement. In biphenyl, the fluorescenc e decay curves vary near the transition temperature T-c = 40 K. It is attri buted to the increase of the excimer-like acceptor near T-c. The analysis o f the absorption band tail shows that the tortional soft mode at 25 cm(-1) is the main origin of the deviation from the simple Urbach-rule below T-c. In p-terphenyl, the sharp peaks due to tortional modes in the excited state are observed near the absorption edge and they disappear above similar to 50 K. (C) 2000 Elsevier Science B.V. All rights reserved.