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

Abstract

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 Tm, the SFG contributions from both interfaces (PS/sapphire and alkane/PS) are comparable and we observe strong interference effects. At temperatures below Tm, 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 accurate...