Infrared reflection-absorption spectroscopy of thin organic films on nonmetallic substrates:: Optimal angle of incidence

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 degrees. The peak intensity of the asymmetric CH2 and COO- stretching bands was measured on the difference, (Delta R) and the normalized (Delta R/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 Delta R and Delta R/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 degrees 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 molecular orientation is at least as good at high angles of incidence than near the Brewster angle.