Numerical simulation of the infrared spectra of thin organic films

Abstract

We present results of numerical simulations of the infrared spectra of thin films of behenic acid (b.a.) in different spectroscopic configurations. A model of independent oscillators was used to derive the optical constants of the material in the infrared region and, from these, the reflectance expressions corresponding to several experimental situations. In this manner, the theoretical attenuated total reflection spectra could be fitted to the experimental result through a steepest-descent routine, and the spectroscopic parameters so optmized were used in the numerical computation of the behavior of the optical constants and of the different spectra of interest. We present results for both Langmuir-Blodgett thin films and thicker evaporated samples of b.a. deposited on ZnSe crystals. In the former case, we also analyze the best suited experimental configuration for the thickness estimate of the films. We have shown that the preliminary use of numerical simulation techniques can simplify the investigation of the spectroscopic properties of thin organic films by allowing the operating limits and capabilities of each experiment to be estimated. Since fatty acid multilayers are the prototypal organized molecular structures, we expect that these techniques become specially important in the spectroscopic investigation of molecular order in thin organic films.