Polarized Raman spectra of thin films. III. Conformational analysis of Langmuir–Blodgett multilayers (1 to 49) of barium stearate

Abstract

Polarized Raman spectra of Langmuir–Blodgett multilayers of barium stearate have been studied in the 2800–3000 cm−1 region (C–H stretching vibrations). The samples have been excited by evanescent waves produced by the plasmon illumination method. Only TM excitation can be used for very thin films. Therefore the polarized Raman spectra are complex, but, due to the smallness of some nondiagonal terms of the Raman tensor, these spectra are similar to the pure polarized spectra given by the TE excitation. When the film thickness decreases from 122.5 to 22.5 nm, the spectra are only slightly modified. The study of the ν(C–C) lines in the 1000–1200 cm−1 region have shown that the chains are still all-trans plane. With ultrathin films (<20 nm) changes are observed in the shape and in the polarization of the 2800–3000 cm−1 band. The first modification (appearance and disappearance of shoulders or peaks) is attributed to conformational change of the acyl chains. The second modification (unpolarization) is related to the deformation of the monolayer deposited on the prism surface which is rough relative to the molecular dimensions. It seems that only the first three layers are concerned by this deformation.