Interference Effect from Buried Interfaces Investigated by Angular-Dependent Infrared−Visible Sum Frequency Generation Technique

Infrared-visible sum frequency generation spectroscopy (SFG) in conjunction with total internal reflection geometry (TIR) has been demonstrated as a powerful technique to study buried polymer interfaces. We have developed a theoretical model using linear and nonlinear boundary conditions to calculate the SFG signals as a function of incident angles and thickness of the polymer films. The validity of this model is tested using a polystyrene film (PS) coated on a sapphire prism. This PS film is exposed to heneicosane (C21H44) above and below its melting temperature. At temperatures greater than T m , the SFG contributions from both interfaces (PS/sapphire and alkane/PS) are comparable and we observe strong interference effects. At temperatures below T m , the SFG signals are dominated by the methyl signals of all-trans heneicosane molecules at the alkane/PS interface. The theoretical model is able to accurately capture the angle and thickness dependence of the SFG signal and provides a valuable tool to accurately determine the interference effects in multilayer samples using SFG in total internal reflection geometry. The model also provides physical parameters (i.e., film thickness, incident angle and substrate index of refraction) needed to suppress or enhance SFG signals generated at a particular interface.