Molecular orientation, aggregation, and order in rhodamine films at the fused silica/air interface

We report optical second harmonic generation studies of the organic dye mol ecule rhodamine 6G spin cast on fused silica surfaces. The concentration de pendence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer sur face coverage. This behavior reflects the formation of ordered molecular ad layers which persist fur approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orienta tion within adjacent layers to be determined. Optical absorbance measuremen ts of the films indicate the onset of rhodamine 6G aggregate formation at s urface coverages of approximately one monolayer. However, the onset of dime r or aggregate fluorescence is observed to occur only at much higher surfac e coverages, consistent with the loss of orientational order within the adl ayers. Our results indicate strong adsorbate-substrate interaction which gi ves rise to orientational anisotropy within the first molecular layer. Well -defined order within subsequent layers is determined by interlayer adsorba te-adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfac ial ordering at the solid/air interface.