Effect of interfacial refractive index on optical molecular orientation measurements

The sensitivity of optical molecular orientation measurements to assumptions regarding thin film refractive index was investigated. Specifically, the influence of the interfacial refractive index on second harmonic generation (SHG) and linear dichroism measurements made in a total internal reflection (TIR) geometry was probed for five distinct molecular systems. The five molecular thin films ranged from weakly adsorbed species in equilibrium with solution to covalently bound molecules. Polarization data from the two techniques were fit using a range of assumed interfacial refractive indices. Surprisingly, a linear relationship between the difference in calculated apparent orientation angle and the difference in solvent-prism refractive index was observed. The trend indicates that for a TIR geometry, the error introduced by the thin film refractive index is negligible when the difference in solvent and prism refractive indices is less than approximately 0.08. However, there are clearly cases, such as a glass/air interface, in which assumptions regarding the thin film refractive index can result in significant error in the extracted orientation angle.