Knowledge Bank

An important problem in modern materials chemistry involves the structure and order of molecules at interfaces between materials of differing composition. These systems comprise an especially challenging problem to the molecular spectroscopist, since there are typically only a small number of molecules exerting an inordinately large influence on the properties of the structure, the concept of order is far different than it is in macroscopic systems, and the interface may be buried and not available for direct examination. These problems have been attacked in our laboratory by exploiting the special properties of integrated optical structures and coupling them to a variety of different spectroscopic probes. Orientations of molecules at dielectric surfaces can be obtained from careful measurements of linear dichroic ratios excited by high-order eigenmodes in Ti:Zn glass waveguides. These measurements have been applied while perturbing monolayers with various external forces and have taught us a great deal about the response of two-dimensional molecular systems to these perturbations. Raman scattering from monolayers of molecules at dielectric surfaces has been achieved in composite systems comprised of layered oblate noble metal ellipsoids and sputtered oxides. These experiments have taught us a great deal about intermolecular interactions and their response to external perturbations. A new set of measurements is using fluorescence anisotropy in these monolayer systems to characterize the distribution of orientations obtained. Finally by paying careful attention to the electric field amplitude distributions in integrated optical structures, it has even been possible to perform optical depth profiling experiments. Despite the successes achieved through the use of integrated optical structures, a number of very challenging and important problems remain to be solved. For example, much of what is currently known about the interaction of light and molecules on surfaces rests on the use of a macroscopic description. A microscopic description of the local field present at an interface is a necessity to obtain relaible absolute measurements of structure. In addition we are still lacking a probe which has the power of nmr for determining intermolecular interactions at interfaces. Thus, while much has been learned, there remains much to do.