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 molecule rhodamine 6G spin cast on fused silica surfaces. The concentration dependence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer surface coverage. This behavior reflects the formation of ordered molecular adlayers which persist fur approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orientation within adjacent layers to be determined. Optical absorbance measurements of the films indicate the onset of rhodamine 6G aggregate formation at surface coverages of approximately one monolayer. However, the onset of dimer or aggregate fluorescence is observed to occur only at much higher surface coverages, consistent with the loss of orientational order within the adlayers. Our results indicate strong adsorbate-substrate interaction which gives rise to orientational anisotropy within the first molecular layer. Well-defined order within subsequent layers is determined by interlayer adsorbate-adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfacial ordering at the solid/air interface.