Waveguide linear-dichroism measurements were used in determining the molecular orientation of N-(sulfopropyl)-4-(p-dioctylaminostyryl) pyridinium in Langmuir-Blodgett monolayers upon 150-microm-thick glass (Ti:Zn) substrates in real time. Acquisition of complete spatial-decay curves in t - 1 s with P(laser) = 100 microW was made possible by combining propagation in integrated-optical structures with direct imaging of monolayer or glass fluorescence onto the active area of a charge-coupled device camera. Multichannel detection permitted measurement of optical waveguide loss coefficients and ultimately of molecular orientations in approximately 10(-3) of the amount of time required by previous methodology. The effects of sample preparation and materials processing on the accuracy and precision of dichroic ratios are discussed. Observed dichroic ratios for a hemicyanine dye revealed no measurable change in the orientation of the electronic transition-dipole moment when Langmuir-Blodgett films were deposited at different surface pressures. These results are in good agreement with the results from singly resonant second-harmonic generation experiments.
Modern surface science is now emerging into a regime in which understanding the relationship betweeen structure and function will occupy an ever more prominent role. To this end one important piece of information concerns the three dimensional orientation of complex adsorbates at an interface. We have been using total internal reflection excited surface Raman scattering and waveguide mediated linear dichroism to measure the tilt angles of molecules adsorbed under realistic conditions (i.e. at the solid-liquid interface). These efforts will be described and compared.
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