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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High-resolution spectra from 20 to 200 cm(-1) for temperatures from 10 to 300 K give interference fringes that cannot be fitted by previous analysis in terms of four Lorentz oscillators. Parameters in the theory are revised to give a better fit both to the far IR and to the reststrahlen spectra.

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The anharmonicity of the infrared active lattice modes of sodium nitrate is studied by measurements of the temperature dependence of the reflectivity. The infrared reflectivity is recorded from 10 to 600 cm-1, in the temperature range 23 to 300 "C, and analysed with the four-parameter semi-quantum model. The comparison between the results of the quantum theory and the observed dependence of damping shows that cubic anharmonicity drives the lattice dynamics of sodium nitrate in the low-temperature phase. Damping divergence is observed for several modes in the vicinity of the phase transition temperature.L'anharmonicite des modes de rBseau actifs en infrarouge dans le nitrate de sodium, a BtB 6tudiee Q partir de 1'6volution du facteur de reflexion avec la temperature. Le facteur de reflexion dans l'infrarouge a BtB mesure de 10 B 600 cm-l, Q des temperatures comprises entre 23 et 300 "C, puis analyse B l'aide du modkle semi-quantique B quatre param6tres. La comparaison entre les resultats de la theorie quantique e t 1'6volution de la fonction amortissement avec la temperature montre que I'anharmonicitt5 cubique regit la dynamique d u rkseau du nitrate de sodium dans la phase de basse tempkrature. L'amortissement de plusieurs modes diverge au voisinage de la temperature de transition.

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Among the three translational frequencies of these monoclinic molecular crystals, two are expected when the electric field of the radiation is parallel to the monoclinic b axis, and one when the field is perpendicular to b. The infrared absorption spectra show that the last band is obtained in each case for an electric field parallel to the easy cleavage plane, at 66 cm−1 for naphthalene, 65 cm−1 for durene, 61 cm−1 for anthracene. This polarization is explained for naphthalene in terms of the tensor of polarizabilities of the molecule, and of its orientation in the crystal. For E parallel to the monoclinic axis the two expected absorption bands are found at 37 and 68 cm−1 for durene, 101 and 120 cm−1 for anthracene, and around 96 cm−1 for naphthalene where they have not been separated. All these bands are shifted towards higher frequencies when the crystal is cooled down to liquid-nitrogen temperature.