Infrared Reflection−Absorption Spectroscopy of Thin Organic Films on Nonmetallic Substrates: Optimal Angle of Incidence

Abstract

Infrared external reflectance spectra of a Langmuir−Blodgett film of cadmium arachidate deposited on a glass substrate were recorded for s- and p-polarized radiations at different angles of incidence from 20 to 80°. The peak intensity of the asymmetric CH2 and COO- stretching bands was measured on the difference (ΔR) and the normalized (ΔR/R) spectra and compared quantitatively with that simulated using a three-phase model system consisting of air, a uniaxial film, and a semi-infinite glass substrate. The maximum intensity in the ΔR and ΔR/R spectra occurs at different angles of incidence. However, when the signal-to-noise ratio is taken into account, the optimal incident angle for both the difference and the normalized spectra is at about 75° for the p- and s-polarizations. Reflectance spectra at different angles of incidence have also been studied as a function of the orientation of the hydrocarbon tail relative to the surface normal. The results show that the accuracy of the determination of the molecu...