The results of Raman scattering and infrared reflectivity measurements on the IV-VI layer-type semiconductors SnS and SnSe are presented. The infrared-active TO, the associated I.O-phonon frequencies, and the dielectric constants for all three principal polarizations have been determined from a Kramers-Kronig analysis of the reflectivity data. The symmetries of the zone-center phonons observed in the different polarization configurations are in agreement with the group-theoretical analysis of the D, '"space group of these compounds. Despite the center of inversion symmetry in this structure, some infraredand Raman-active modes are found to be nearly degenerate, suggesting the importance of the layerlike character in these compounds as in the isomorphic GeS and GeSe. A comparison of the phonon frequencies of the corresponding modes in the spectra of SnS and SnSe, or GeS and GeSe, indicates that the frequencies vary as a power ( -2.2) of the lattice constant.

We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule all with distinctly different backbone conformations. The organic materials investigated here are planar methylated ladder-type poly para-phenylene, semi-planar polyfluorene, and nonplanar para-hexaphenyl. In the long-chain-polymers the photoluminescence transition energies blueshift with increasing temperatures. In contrast, in the conjugated molecules transition energies redshift with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors are extended to explain the temperature dependence of the transition energies in conjugated molecules.

We present experimental and theoretical findings on the geometry of polycrystalline para hexaphenyl via Raman scattering. The planarity of the molecule is affected by hydrostatic pressure and temperature.Our studies indicate that the potential energy curve which governs the torsional motion between neighboring phenyl rings is "W" shaped. We determine the activation energy to promote the molecule from a nonplanar to a planar state to be 0.04 eV, in good agreement with our quantum chemical calculations. From the relative intensities of the 1280 cm 21 to the 1220 cm 21 Raman modes we show that high pressure planarizes the molecules, modifying the "W"-shaped potential energy curve to a "U"-shaped one.[S0031-9007(99)09073-0]