Search

Optical second harmonic generation (SHG) studies of malachite green (MG) adsorbed at the fused-silica/air interface have been conducted. The concentration dependence of the second harmonic intensity indicates the formation of ordered monolayers normal to the interface. Polarization SHG experiments have been used to extract the averaged molecular orientation in adjacent molecular layers and to characterize the breadth of the orientation distribution. MG adsorbs in an upright orientation with its molecular plane near normal to the surface. This orientation is consistent with the interaction of the polar surface and the polar dimethylamino substituents of the adsorbate. Molecular orientation in the second monolayer resembles that in the first, but also reflects some additional broadening of the orientation distribution function. The onset of parallel, sandwich-type aggregates has been observed within the first monolayer. The formation of parallel, sandwich-type, intralayer aggregates is distinct from the interlayer aggregation observed between adjacent monolayers of rhodamine 6G. These differences result from the nature of the molecule−surface interactions that define their respective orientations in the first monolayer. Thus, it appears that the order observed in these ultrathin organic films is dictated by the molecule−surface interactions within the first monolayer as well as the magnitude of the intermolecular interactions in subsequent layers.

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 for 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.

Stability of vitamin D3on aerosil surfaces has been studied by UV- and IR-spectroscopy. The difference in the thermobehaviour of vitamin D3in the adsorbed state at low and high surface coverages has been observed and discussed in terms of the competition between vitamin D3molecules and their thermoisomers for the adsorption sites.