The far infrared spectra of the copper halides CuCl, CuBr, and Cul have been measured in transmission and in reflection at various fixed temperatures from room temperature to liquid-helium temperature. Marked changes of such parameters as resonance frquency and damping as a function of temperature have been observed, also a line-splitting in the case of CuCl. In addition, a considerable increase in the asymmetry of the transmission curves with decreasing temperature has been observed.

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The reflection spectra of AgCl, AgBr, and AgI are measured at liquid hydrogen, liquid nitrogen, and room temperatures. For AgCl, they show distinctly two components. For AgBr and AgI, the structure, though complex, is not resolved. Nevertheless, in all cases, except for AgI, at low temperature, the Lorentz analysis needs two oscillators. The stronger one v(1) corresponds to the TO transition at point Gamma, the second one v(2), about an order of magnitude smaller, corresponds to a TA + TO addition band for the NaCl type structure, and to a TA + LA for the zinc blende or wurtzite structure, confirming preliminary measurements made by a completely different experimental approach (transmission of thin evaporated films instead of reflection on bulk crystals). They are, however, slightly distorted towards high frequencies. The proposed frequencies for the TO branch at point Gamma are, for AgCl: 105 cm(-1) (290 degrees K), 114 cm(-1), (80 degrees K), 119 cm(-1) (25 degrees K); for AgBr: 79 cm(-1) (290 degrees K), 84 cm(-1) (80 degrees K), 85 cm(-1) (25 degrees K), and for AgI: 103 cm(-1) (290 degrees K), 106 cm(-1) (80 degrees K), 106 cm(-1) (25 degrees K). The oscillator strengths and the damping of both oscillators are studied vs temperature. A large decrease of the damping is still observed between liquid nitrogen and liquid hydrogen temperatures. The refractive index and the absorption index are determined in the whole ir for the first time at liquid hydrogen, liquid nitrogen, and room temperatures, showing large variations, particularly a high transparency in the very far ir at liquid hydrogen temperature. The maximum values of n and k range from 2 to 10. The real and the imaginary parts of the dielectric constants are also computed for the whole ir range at room and lower temperatures down to liquid hydrogen temperature.

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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 diferent 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-diGerence processes; and (4} the 6rst 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 cm, having the same frequencies as the three maxima in the lattice-mode density, which correspond approximately to characteristic TQ, LA, and TA phonons, The effect of other impurities and of neutron irradiation is also considered.