Effect of Interfacial Refractive Index on Optical Molecular Orientation Measurements

Abstract

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.

Microscopic molecular optics theory of surface second harmonic generation and sum-frequency generation spectroscopy based on the discrete dipole lattice model

“…37,42 There are also efforts trying to show that the dielectric constant of the extremely thin monolayer is just the simple arithmetic average of the two bulk dielectric constants. 52,58,67,68 Event though the results from these approaches seemed reasonable, neglecting the anisotropic microscopic local field effect in the molecular monolayer is not likely to be physically sound.…”

“…42,52,58,67,68,69,70,71 In SFG-VS, some considered the film anisotropic and its optical constant is determined using the Clausius-Mossotti relationship with the ellipsometry data of the film.…”

“…They are also close to the estimation made by Zhuang et al using a modified Lorentz model of the interface, 53 and the simple average of the optical constant of the two adjacent isotropic bulk phases. 52,58,68 However, even though these values worked almost fine, the models and the reasonings behind it prevented the treatment of the molecular details of the molecular interface. With the development of the laser techniques and the measurement methodology in the surface SHG and SFG-VS, 21,25,39,40 now such details can be interrogated from the careful experiments.…”

“…21,25,41,42,43,44,45,46,47,48,49,50,51,52,53 It has been well demonstrated that in majority cases the calculation of the molecular orientation is quite sensitive to the values used for these two factors. 21,25,42,51,52 In the currently accepted models of the nonlinear optics of interface, 4,34,41,53,54,55 the nonlinear radiation was treated as the result of an infinitesimally thin polarization sheet layer, and as the starting pint a macroscopic dielectric constant was assigned to this thin layer. 4,34,54,55 This model was a natural extension of the original formulation by Bloembergen and Pershan, where a nonlinear plane parallel slab with a finite thickness embedded between two linear dielectrics was the source of the SHG radiation, and where the Maxwell's equations which satisfy the boundary conditions at the two plane interfaces were accurately solved.…”

Treatment of Linear and Nonlinear Dielectric Property of Molecular Monolayer and Submonolayer with Microscopic Dipole Lattice Model: I. Second Harmonic Generation and Sum-Frequency Generation

“…38,39,40 However, researchers have long realized that the crucial issues in the quantitative interface studies with the SHG and SFG are on how to describe the macroscopic dielectric constant of the molecularly thin layer and how to calculate the anisotropic two dimensional microscopic local field effect. 21,25,41,42,43,44,45,46,47,48,49,50,51,52,53 It has been well demonstrated that in majority cases the calculation of the molecular orientation is quite sensitive to the values used for these two factors. 21,25,42,51,52 In the currently accepted models of the nonlinear optics of interface, 4,34,41,53,54,55 the nonlinear radiation was treated as the result of an infinitesimally thin polarization sheet layer, and as the starting pint a macroscopic dielectric constant was assigned to this thin layer.…”