Optical second harmonic generation study of malachite green orientation and order at the fused-silica/air interface

Optical second harmonic generation (SHG) studies of malachite green (MG) ad sorbed at the fused-silica/air interface have been conducted. The concentra tion dependence of the second harmonic intensity indicates the formation of ordered monolayers normal to the interface. Polarization SHG experiments h ave been used to extract the averaged molecular orientation in adjacent mol ecular layers and to characterize the breadth of the orientation distributi on. MG adsorbs in an upright orientation with its molecular plane near norm al to the surface. This orientation is consistent with the interaction of t he 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 distributi on function. The onset of parallel, sandwich-type aggregates has been obser ved within the first monolayer. The formation of parallel, sandwich-type, i ntralayer aggregates is distinct from the interlayer aggregation observed b etween adjacent monolayers of rhodamine 6G. These differences result from t he nature of the molecule-surface interactions that define their respective orientations in the first monolayer. Thus, it appears that the order obser ved in these ultrathin organic films is dictated by the molecule-surface in teractions within the first monolayer as well as the magnitude of the inter molecular interactions in subsequent layers.