Vibrational spectroscopy of supermolecular assemblies

Vibrational spectroscopy as a tool for ultrathin organic film structural investigations is considered. The application of total-internal-reflection-excited surface Raman spectroscopy to self-assembled and Langmuir-Blodgett monolayers and multilayer structures is described. The substrate is fused silica upon which a metallic mask has been deposited. Laser excitation is on-coupled from the rear through the ~100-μm opening in the mask. A metal island film (Ag or Au) of ~50-Å thickness is evaporated onto the substrate and is then covered with a sputtered dielectric layer. Typical results for a quaternary ammonium cationic amphiphile are shown. Two types of information can be obtained from the spectrum. The depolarization ratios can be used, at least in principle, to determine the orientation of the polarizability ellipsoid of the molecule, and spectral interpretation can be used just as in bulk systems