A double polarization modulation far‐infrared spectrometer

A new far‐infrared spectrometer design is described that utilizes double polarization modulation. Based on the Martin–Puplett interferometer, this new design has high sensitivity in the region between 5 and 200 cm−1 and eliminates problems due to fluctuations in the source, interferometer drift, and reduced dynamic range in studies with small sample absorption. The new spectrometer allows, for the first time, the extension of reflection‐absorption spectroscopy of ultrathin films and surface coatings into the far infrared. We present far infrared reflection‐absorption data of ultrathin potassium iodide films and submicron thick polyethylene oxide films. Use of the spectrometer for far infrared dichroism studies of thin polymer films and dielectric crystals and polarization‐dependent normal incidence reflectance measurements is also described.