Milton Gottlieb

Optical Properties of Lithium Fluoride in the Infrared

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.

Reflectivity of Li 6 F and Li 7 F

Reflectivity measurements were performed on films of Li6F and Li7F. The films were deposited onto substrates of natural lithium fluoride crystals by vacuum evaporation. Typical film thickness was 10 μ. Because the lattice constants of the pure isotope material and the natural crystal differ by less than 0.03%, the crystallinity of the films was good. It was found that the fundamental optical mode frequency is proportional to the square root of the reduced mass, as predicted from the simple model of an ionic crystal.