FLUORESCENCE, STUDIES ON STRUCTURAL PHASE-TRANSITIONS OF MOLECULAR-CRYSTALS

Fluorescence spectra and lifetime of biphenyl single crystals were stu died over a wide temperature range using a pico-second UV laser. In co ntrast to the sharp peaks near 530 nm observed in crystals grown from the methyl alcohol solution, a new band around 500 nm was found in zon e-refined crystals at 5.0 K when the intense exciting laser was employ ed. It was found that the lifetime of the intrinsic fluorescence band reaches its minimum at the incommensurate-commensurate structural phas e transition point only when the 500 nm broad band appears. The concen tration of the accepters is supposed to be increased by the crystal he ld fluctuations near the phase transition and the lifetime shortening is explained by the Forster transition mechanism.