EXCITON REFLECTION SPECTRA OF TETRACENE SINGLE-CRYSTALS

The exciton reflection spectra of free tetracene single crystals and those in optical contact with different substrates (quartz, glass, muscovite) are measured in polarized light at temperatures ranging from 300 to 4.2 K. Peculiarities in the reflection spectra of a free crystal are due to a phase transition in the crystal at 160 K. The phase transition is shown to result from an increase of the crystal internal pressure due to the crystal stress with lowering temperature and leads to the change of the crystal symmetry. The b‐polarized reflection spectra of free and in‐contact crystals show a structure which is related to the exciton effect in the surface region. A weak exciton‐phonon coupling in tetracene is concluded. Copyright © 1979 WILEY‐VCH Verlag GmbH & Co. KGaA