Optical Constant Determination in the Infrared of Uniaxially Oriented Monolayers from Transmittance and Reflectance Measurements

Anisotropic optical constants (index of refraction and extinction coefficient) of Langmuir-Blodgett (LB) cadmium arachidate (CdAr) monolayers have been determined from experimental FTIR spectra. The inplane and out-of-plane complex refractive indexes were calculated using an iterative procedure from a normalized transmittance spectrum at normal incidence and a p-polarized reflectance spectrum at grazing incidence, respectively. The in-plane refractive index (n^x=n^y) has been obtained for 8 and 10 CdAr monolayers deposited on calcium fluoride, zinc selenide, and silicon substrates. On the other hand, the out-of-plane refractive index (n^z) has been determined for 7 CdAr monolayers deposited on a gold substrate. Orientation of CdAr molecules has been evaluated from the anisotropic extinction coefficients of the ν_aCH₂ and ν_sCH₂modes; a tilt angle of about 12° has been found for the CdAr alkyl chains, which is in good agreement with the values available in the literature. Finally, based on these optical constants, a PM-IRRAS spectrum of a CdAr monolayer at the air/water interface has been simulated by using an anisotropic multilayer formalism. The simulated and experimental spectra have been compared in order to reveal the differences in both the orientation and the organization of the CdAr molecules deposited on solid and liquid substrates.