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 c admium 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- stretch ing bands was measured on the difference, (Delta R) and the normalized (Delta R/R) spectra and compared quantitatively with that simulated u sing a three-phase model system consisting of air, a uniaxial film, an d a semi-infinite glass substrate. The maximum intensity in the Delta R and Delta R/R spectra occurs at different angles of incidence. Howev er, 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 no rmal. The results show that the accuracy of the determination of the m olecular orientation is at least as good at high angles of incidence t han near the Brewster angle.