Optical properties of cooled Rhodamine B in ethanol

Abstract

The optical properties of an organic laser dye luminophor, Rhodamine B in ethanol, were investigated at both high and low concentrations as a function of temperature. At high concentrations the absorption spectra (extinction coefficient, k) and the refractive index (n) were determined by measuring the reflectivity of parallel-polarized light at the pseudo-Brewster angle at the dye–air interface. The absorption spectra at low concentration were also measured and compared with those at high concentration. It was found that at high concentration, independent of temperature, the dye–dye interaction among neighboring dye molecules contributes to the deviation of the absorption spectra from Beer's law. The dependence of absorption spectral distribution on concentration becomes more pronounced as the solvent temperature is reduced. This is thought to be mainly due to the mutual interaction of neighboring molecules and also to the formation of higher-order aggregates. At low concentrations, as the temperature is reduced, the absorption spectra show little change and, in contrast to those at high concentration, obey the Beer–Lambert law; i.e., the absorption cross sections are independent of concentration. The absorption intensity of concentrated solutions decreases markedly compared with those at low concentrations.