The Polarized Absorption Edge and the Davydov Splitting of Anthracene

Abstract

The temperature dependence of the absorption edge of crystalline anthracene near 4000 Å has been re-examined using the Glan-Thomson prism in the temperature range from 390 to 79°K. The long wave-length tail of the first absorption band and that of the absorption shoulder due to the vibrationally allowed transition are both represented by the Urbach rule: for the first absorption band σ_a′=1.73 and σ_b′=1.76; E_0a=3.168 eV and E_0b=3.123 eV. The absorption edge of the second absorption band near 2700 Å is also studied, where the b component has an exponentially rising edge as if governed by the Urbach rule.
The Davydov splitting is investigated on doped and undoped thin flakes, and the polarization ratio is measured on bulk crystals. The smaller splitting is discussed in connection with thermal libration of molecular axes or lattice distortion induced by the substituted tetracene molecules. In the pure crystal the first absorption peak is split by about 310 cm⁻¹ for 0°K.