Polarization Modulation FT-IR Spectroscopy

Several infrared absorption measurements such as molecular orientation, thin films on reflective surfaces and crystal structure studies are enhanced with the use of polarized light. Sequentially collected sample and reference spectra are measured using infrared polarizer accessories. Infrared Reflection Absorption Spectroscopy (IRRAS) uses p-polarized light at a grazing angle of incidence to measure thin films on metallic substrates. The reflectance measurement is done at a grazing angle to increase the polarized absorption from the thin film. Often the absorptions from these samples are relatively weak, as with molecular monolayers, or possibly masked by interstitial absorptions from the sample. Subtraction techniques may be used for some experiments to eliminate sample matrix absorptions, but often the weaker polarization effect is lost in the result. When the sample is a monolayer, the surface sensitivity of the IRRAS technique may not be sufficient even if spectral information is signal averaged over a long period of time. The greater the time separating sample and background data collections, the larger are the effects of theminute instabilities of the spectrometer atmosphere and detection system, resulting in relatively large spectral contributions from carbon dioxide and water. These atmospheric absorptions make the measurement of the sample impossible. With simultaneous collection of differentially polarized signals in the PM IRRAS (Polarization Modulation – Infrared Reflection Absorption Spectroscopy) technique, this problem is eliminated. This note reviews the essentials of this simultaneous, dual-channel technique and then introduces an application of the technology