Optical Properties of Lithium Fluoride in the Infrared*

Abstract

Reflectivity measurements were performed on lithium fluoride single crystals at 135°K, 210°K, 300°K, and 355°K at wavelengths from 2 μ to 60 μ. A Perkin-Elmer 112 prism monochromator was used between 2 μ and 30 μ and was modified to a grating monochromator for use between 30 μ and 60 μ. The reflection band near the reststrahl peak shows pronounced secondary structure. A Kramers-Kronig dispersion relation was applied to the Fresnel reflection formula, and the secondary structure was exhibited in the optical constants. The analysis was performed on an IBM-704 computer, and the phase of the reflectivity was determined at some 400 points in the 2 μ to 60 μ range. For each of these points, the real and imaginary dielectric constants, the index of refraction, the extinction coefficient, and the absorption constant were determined. In addition to the main peak of the imaginary dielectric constant at 312.5 cm−1, two secondary peaks appear on the short wavelength side of the maximum. One is at 485 cm−1 and becomes sharper at higher temperature; the other is at 650 cm−1 and is relatively insensitive to temperature over the range measured. An interpretation based on the coupling of the fundamental longitudinal mode to the radiation field is discussed. The damping constants of the main peak were evaluated and were found to vary linearly with temperature over the range measured.