The infrared absorption spectra of polycrystalline films of C2H2, C2HD, and C2D2 have been examined at 63°K in the frequency range between 4500 and 450 cm—1. The spectra of the solid state and the gas phase of the various acetylenes are compared and the appropriate vibrational assignments are made. Several absorptions attributable to combinations of lattice modes and molecular fundamentals were found. The observed multiplet structure of the fundamental vibrations indicates that in the low-temperature modification of C2H2 and C2D2 the molecules are located at sites of C2h symmetry. Spectra of various solid solutions composed of mixtures of isotopic species were obtained and their significance discussed. The effect of isotopic substitution on the splittings in the ν5 region led to the conclusion that the dipole—dipole coupling model is sufficient to explain the observed spectra.
The polarized laser-Raman scattering of oriented single crystals of β-AgI has been investigated at 300°, 80°, and 4.2°K. The fundamental optic lattice modes predicted by group theory were observed and assigned on the basis of scattering tensors. Further support for the present assignments of the β-AgI optic lattice modes comes from the approximately linear relations found between the squares of the optic-mode frequencies of a number of wurtzite-type crystals and their respective reciprocal reduced masses. The frequencies of the two silent optic lattice modes were determined by a normal coordinate treatment.
The infrared transmission spectra of AgCl, AgBr, and AgI have been measured between 400–30 cm−1 at room temperature and at 143°K. Strong absorptions characteristic of the transverse optic modes were found, together with weak subsidiary absorptions arising from multiphonon combinations. The longitudinal optic modes of silver chloride and silver bromide were obtained from the reflection spectra of thin, silver halide films deposited on a silvered substrate. Capacitance measurements were used to determine the static dielectric constant of AgI; ε0=7.0±0.3. For a series of isomorphous I—VII compounds having large reduced masses, approximately linear relations are found between νt2 and the reciprocal reduced masses. The Szigeti relations for effective charge and compressibility were calculated and their significance is discussed.
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