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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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.

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