Optical analysis based on spectral transmittance or absorbance by liquid samples with application to Dodecane oil and dissolved Nile Red dye

Abstract

Transmittance or absorbance spectra of liquid samples are usually obtained by illuminating a quartz cuvette container with non-polarized light. A direct inversion of the absorbance allows to obtain the optical thickness and the absorption coefficient, as well as the intrinsic absorptivity of the medium when the concentration of the light absorbing molecules is known. A numerical inversion of transmittance spectra, based on a spectral projected gradient method, is carried out to obtain other optical parameters: optical constants (refractive index and extinction coefficient) and electronic polarizability of the molecules. This inversion approach is applied to transmittance spectra by samples of transparent Dodecane oil and light absorbing Nile Red dye, with an automated determination of the HOMO-LUMO energy gap of saturated Nile Red dissolved in Dodecane oil. The spectral variation of the optical constants of these two materials is reported for the first time, for wavelengths between 300 and 800 nm.