Abstract

We have elucidated the microstructural properties of a monolayer of a cyanine dye, S120, deposited on a glass substrate by the Langmuir-Blodgett deposition technique. This system is known to form ellipsoidal shaped aggregates of the order of 100 μm in diameter.¹ Although these aggregates were previously believed to be isotropically distributed on the water surface of the Langmuir-Blodgett trough and, therefore, on the glass substrate to which they are subsequently transferred, we demonstrate that their distribution function has a two-fold axis of symmetry. This shows up clearly in the orientational dependence of the second harmonic generation (SHG) signal from the film. We provide evidence that this anisotropy originates with an ordering of the elliptical aggregates during the isothermal compression phase on the water surface. By combining the polarization and orientational properties of the SHG signal from the film,² we deduce for the first time that the individual aggregates have C₂ crystal symmetry, in agreement with the proposed brickstone model.¹ This was possible even though the laser beam (2.3-mm diameter) averaged over a large number of aggregates and so sampled a distribution of orientations of the aggregates. Had the microscopic distribution been isotropic, determination of the microscopic crystal structure would have been ambiguous. This suggests that a deliberate reduction in the symmetry of an otherwise isotropic distribution could be useful in other systems.

© 1988 Optical Society of America