Urbach Tail of Organic Dyes in Solution

Abstract

Emission excitation spectra have been measured in rhodamine 6G and tetracene in solutions at various temperatures in the low-energy side of the lowest absorption band. It has been found that the spectrum of rhodamine 6G is well approximated by a Gaussian convoluted by an exponential tail, while that of tetracene has shoulders in the nearly exponential tail. The temperature dependence of the low-energy tail of rhodamine 6G has been compared with that of the Urbach tail in solids. A long exponential-like tail has also been found in the emission spectrum of rhodamine 6G over ∼5000 cm⁻¹ toward the low-energy side. From the analysis of the absorption, excitation and emission spectra, it is shown that both the molecular vibrational modes and the classical mode of the surroundings which has only quadratic interaction with the dye molecule contribute to these tails.