This note describes the extension of internal reflection spectroscopy into the far-infrared region below 150 cm<sup>−1</sup>. To the best of our knowledge this is the first reported use of internal reflection techniques below 250 cm<sup>−1</sup>, the cutoff point of the widely used KRS-5 reflector plate material.
G. L. Carlson and W. G. Fateley, "Internal Reflection Spectroscopy in the Far-Infrared Region," Appl. Spectrosc. 23, 374-375 (1969)