Polarized infrared attenuated total reflection spectroscopy for three-dimensional structural analysis on long-chain compounds

Abstract

Two simple techniques using single-crystal specimens in polarized infrared attenuated total reflection (ATR) spectroscopy were developed for structural studies of molecular crystals, in particular of long-chain compounds. The first technique relates to a rotatory change of sample setting around the normal of the sampling plane of an internal reflection element (IRE) in the p-polarized ATR measurement. This is a simple method to distinguish whether a transition moment is parallel to a certain crystal face. The inclination manner of hydrocarbon chains constructing an orthorhombic polyethylene subcell can be easily determined. Another technique relates to the employment of intermediate polarizations between p and s settings of incident infrared radiation, which allows three-dimensional structural analysis. The evanescent electric field is estimated for a simple model system. Linearly polarized incident radiation generates an elliptical electric field in a specimen. By combining a rotatory change of sample setting, the long axis of the elliptical electric field can be directed in any direction in a fixed coordinate system located on a specimen. Transition moments nearly parallel to the long axis are emphasized in spectra. The phase difference between the p and s components of evanescent waves has a large influence on the elliptically polarized electric field. For a multiple reflection IRE, the polarization of ATR spectra depends on the sampling position, which can be ascribed to a phase jump of the guided infrared radiation at every reflection.