Infrared polarimetry using attenuated total reflection

The theoretical and experimental foundation for using an attenuated total internal reflection (ATR) device in conjunction with a Fourier Transform Infrared Spectropolarimeter (FTIRSP) to determine the full Mueller matrix of highly absorbing materials was developed in this research. It is possible to calculate the linear dichroism (linear diattenuation), circular dichroism (circular diattenuation), linear birefringence (linear retardance), and circular birefringence or optical activity (circular retardance) with the measurement of the Mueller matrix. This research uses the short effective path length via the evanescent field provided by the ATR coupled with the FTIRSP to provide a method for determining the polarization characteristics of relatively opaque organic and inorganic materials. Many researchers are using polarimetric techniques to reveal the structure of biological membranes and films; however, determining the orientation of molecules in these membranes and films requires a knowledge of the complex refractive index of the constituents at the measurement wavelength. High absorbance at the measurement wavelength makes the determination of the complex refractive index difficult; thus, average values measured at nearby wavelengths are often used. The research reported here provides a method for using a spectropolarimeter to determine the complex refractive index over the 3–14 μm infrared spectrum from Mueller matrix data. The circular retardance and circular diattenuation measurements obtained from the Mueller matrix can be used to detect optical activity. Right and left enantiomers can be differentiated by their polarimetric properties. This equipment is used to demonstrate the identification of right and left enantiomers using the equations derived in this effort.