A variable-length reflection cell is employed in the free-space determination of the complex index of refraction of liquids at mm wavelengths. Explicit expressions are obtained for both the ordinary index of refraction η and the extinction coefficient K.
The results of measurements on toluene, dioxane, and cyclohexane are reported for wavelengths of 4.2 mm, 3.2 mm, 2.5 mm, and 1.8 mm.
CHARLES PINE, WILLIAM G. ZOELLNER, and JOHN H. ROHRBAUGH, "Measurement of the Complex Index of Refraction of Liquids below 5-mm Wavelength," J. Opt. Soc. Am. 49, 1202-1206 (1959)