ELECTROOPTICAL SPECTROSCOPY ON AGGREGATED CHROMOPHORE SYSTEMS IN LANGMUIR-BLODGETT-FILMS

Linear and quadratic Stark spectra were measured on LB films that contained a hydrophobically substituted cyanine chromophore in binary mixtures with arachidic acid. The external field was applied normal or parallel to the film surface on samples with different internal symmetry. It was found that the orientation of the symmetry axis of the dye molecule is close to the surface normal. A significant number of molecules, 5-10%, are flipped head-to-tail from the main orientation. Parallel to the film surface, large polarizability changes on electronic excitation, DELTA-alpha approximately 1000 angstrom3, were observed that reflect electron delocalization over aggregates. At low dye concentrations, large internal electric fields, F(int) approximately 2000 kV/cm, were measured that are due to co-adsorption of Cd2+ counter-ions in the deposition process.