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The GETS method (germination epitaxique dans des trous submicroscopiques) needs only a drop of TGS or TGSe solution in heavy water to give a single crystal film. The thickness e (e ≃ 1 μm) is perfectly suited for mid-ir spectroscopy with polarized radiations. New lines are observed. In opposition to previous studies, there is no observation of lines disappearing at the Curie temperature, but several lines are continuously broadening when the temperature is increased from 80 K to 300 K.

Absorption spectra at liquid helium temperature in the far infrared are given for thick plates of CsI, CsBr, TlBr, TlCl, KI, NaCl, InSb, KBr, CuCl, Al_2O_3, SiO_2, MgO, and LaCl_3. For a thickness of about 3 mm all these crystals, completely opaque at room temperature, become quasitransparent at very low temperature. This supertransparency of crystals at low temperature gives several far infrared filters with remarkable cutoffs and a lot of available materials to the spectroscopist in a part of the spectrum where only quartz and polyethene were usable up to now. This is also evidence that the main part of the absorption at room temperature comes from the lattice phonons. It decreases with their number when the crystal is cooled (2 phonons difference process). The small absorption which remains in a few cases—KBr, —NaCl, KI—could arise from 2 phonons addition process nearly independent of temperature and much less efficient to give absorption. In the case of glasses the absorption coefficient is higher by 2 or 3 orders of magnitude and keeps constant when the glass is cooled. For quartz glass, the index of refraction also seemed invariant. An explanation is proposed in terms of one single phonon process, independent of temperature and allowed by the disorder characteristic of glassy states.