Far-infrared reflection-absorption spectroscopy of ultrathin films

Using a new far-infrared spectrometer design, we have extended the technique of reflection-absorption spectroscopy into the far-infrared and millimeter-wave spectral regions. The spectrometer allows the far-infrared spectroscopy of ultrathin films and surface coatings. From an analysis of the electrodynamics of the reflection-absorption technique, a model has been developed that yields the frequency-dependent optical constants from the reflection-absorption spectra and that quantitatively describes the mode asymmetries, frequency shifts, and distortions of band intensity seen in reflection-absorption spectra. We present far-infrared reflection-absorption spectra of potassium iodide films as thin as 150 {}, which demonstrate the utility of this new spectroscopy in exploring systems previously difficult or impossible to study by conventional far-infrared spectroscopy.