Surface and bulk exciton transitions in the reflection spectrum of tetracene crystals

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Abstract
The reflection spectrum of the 0–0 component of the first singlet transition of tetracene crystal has been measured for temperatures ranging from 300 to 1.7 °K. A discontinuous change in the Davydov splitting occurs near 186 °K signifying the occurrence of a phase transition. At the lowest temperatures two anomalous reflectivity minima are observed inside the b‐polarized stopping band that are very similar to structure in the corresponding reflection band of anthracene crystal. The reflectivity minima are tentatively assigned to transitions to surface exciton states lying approximately 5 cm−1 and 160 cm−1 above the b‐polarized exciton transition at 5349 Å. The other structure within the reflection band is interpreted as a coupling of the polariton to vibrational modes of intra‐ and intermolecular origins.