Optical Constants of Potassium Bromide in the Far Infrared

One of the fruits of the recent developments in far-infrared techniques has been the knowledge of the whole vibrational spectrum of a solid and its evolution with temperature. We consider here the particular case of potassium bromide, which has been studied elsewhere by neutron diffraction, and we give all the information obtained in the laboratory by the different methods of far-infrared spectrometry now available: (1) the reflectivity of potassium bromide at liquid-helium, liquid-nitrogen, and room temperature, including the effect of polarization, grazing incidence, and the quality of the surface, and interpreted according to a Kramers-Kronig analysis and a Lorentz analysis with two linear damped oscillators; (2) the transmission of thin layers at normal and oblique incidence, and a determination of the complex index of refraction, the compressibility coefficient, and the effective charge of the ions; (3) the very-far-infrared transmission of thick samples and its temperature dependence explained by phonon-difference processes; and (4) the first spectroscopic observation of the phonon spectrum of an ionic crystal, provided by sodium-induced absorption in potassium bromide, with peaks at 101, 83.5, and 71.5 ${\mathrm{cm}}^{-1\mathrm{}}$, having the same frequencies as the three maxima in the lattice-mode density, which correspond approximately to characteristic TO, LA, and TA phonons. The effect of other impurities and of neutron irradiation is also considered.