Thickness dependence of absorption of molecular thin films studied using FECO spectroscopy

Multiple beam interferometry, the fringes of equal chromatic order (FECO), was used to investigate the absorption properties of aqueous dye (Rhodamine B) films confined between mica surfaces of crossed cylindrical geometry. A new normalization method for the data analysis, using the transmitted intensities of both odd and even order FECO, was developed to obtain the thickness dependence of the absorption by the film. We found that the Rhodamine B molecules were trapped between the mica surfaces below 15 nm separation distance, and only water was expelled from the confined space upon compression.