Far-Infrared Optical Properties of Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Cd${\mathrm{F}}_2$

The absorption coefficients and indices of refraction of pure Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Cd${\mathrm{F}}_2$ at 300, 200, 80, and 5°K have been measured at all accessible frequencies between 300 Gc/sec and the transverse-optic (TO) frequencies of the crystals. The absorption has been interpreted in terms of 2-phonon difference processes involving high-energy phonons. The indices of refraction can be described by a harmonic rigid-ion approximation to the crystal. The temperature dependences of the long-wavelength dielectric constants and the TO frequencies have been measured and related to the Szigeti expression for the effective ionic charges.