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The phonons of deuteriated neopentane C(CD 3 ) 4 single crystals in the orientationally disordered phase were measured at T=173 K by coherent inelastic neutron scattering, yielding very broad bands that spread, at a given energy, over a large portion of the Brillouin zone while sitting on an intense background. No librational mode was detected. Selection rule violations, presumably linked to disorder, were observed. The elastic constants are discussed in terms of rotational-translational coupling, and inelastic scattering results are compared with the molecular center of mass translational disorder as obtained from diffraction experiments providing a confirmation of a previous interpretation of these experiments

The phase transitions of AHOD, studied by inelastic neutron scattering experiments (at OKbar and 5Kbar) and by polarized Raman scattering (at 6.4Kbar), are correctly described by the dynamical coupling between a TA phonon and an orientational pseudospin.

Resume The laser-Raman spectrum of a crystal of monoclinic β sulfur has been recorded at various temperatures above and under the transition point. The frequencies of the internal modes are close to the internal frequencies of α sulfur. The external modes show a behaviour typical of disordered crystals with a progressive ordering of the molecules when the temperature is lowered.

Neutron diffraction experiments performed on orientationally disordered neopentane (C(CD 3 ) 4 ) single crystals are interpreted using a TLS model adapted in order to account as well as possible for the orientational disorder. The molecules are found to be always localized around their most probable orientations at every temperature; the orientational variables display a large, but practically temperature-independent, dispersion with no sign of reorientational dynamics. In contrast, the centre-of-mass displacements, always much larger than what one expects from the phonon contribution, strongly increase at high temperatures Des experiences de diffraction de neutrons effectuees sur des monocristaux de neopentane (C(CD 3 ) 4 ) sont reinterpretees a l'aide d'un modele TLS adapte afin de rendre compte du desordre d'orientation. Les molecules sont localisees autour de leur orientation la plus probable a toutes les temperatures

The ODIC phase of adamantane (208 K-541 K) was studied by Raman light scattering between 209 K and 500 K. The 1 220 cm-1, E symmetry, vibrational line was recorded with Eg and F2g symmetries. The Eg spectra, due to ordered molecules, are broadened by a residence time effect. The F2g spectra, which correspond to molecules as they rotate, could be attributed to the modification of the internal vibration by the environment during the molecular reorientations.

Neutron diffraction experiments have been performed on a single crystal of the high-temperature phase of KClO 4 at 325 o C and 420 o C. The analysis of the structure factor of Bragg reflections leads to the static orientational probability density function. The positions of the maxima of this function show that ClO 4 − ions have preferred orientations of T d symmetry, with two possible positions. Comparison is made of this result with previous papers. It is also suggested that the ions could reorient between the two potential wells mainly by non-instantaneous rotations around the four-fold axes of the cube Etude d'un monocristal de KClO 4 , dans sa phase a haute temperature, par diffraction de neutrons a 325 et 420 o C. Determination de la fonction statique de densite de probabilite d'orientation a partir de l'analyse du facteur de structure des reflexions de Bragg: les positions des maximums de cette fonction indiquent des orientations privilegiees de symetrie T d , avec deux positions possibles pour les ions ClO 4 − , qui pourraient effectuer des rotations non instantanees autour des axes quaternaires du cube

The vibrational spectra of single crystals of cesium triiodide and rubidium triiodide have been recorded at 120 and 40°K. From polarisation measurements, bands can be resigned by taking into account the factor group effect. Linear and slightly angular configurations of the I 3 − ion are discussed.

Author(s): Debeau, CM; Silverman, D | Abstract: A model for lepton pair production from quark-antiquark annihilation is presented in which antiquarks are obtained from constituent pions and form factors are included to account for the transverse-momentum dependence. The single-lepton spectrum and the lepton-pair spectra in mass squared, transverse momentum, and longitudinal momentum are calculated and compared with proton- and pion-beam experiments. © 1978 The American Physical Society.

A plastic neopentane 1.5x1.5x4 cm3 single crystal was grown from the vapour at 235 K. This paper describes in detail the design of the apparatus and the growing process. The crystallinity and orientation of the crystal obtained were checked through a Laue X-ray diffraction method, allowing us to overcome the difficulties arising from the low melting temperature, fast sublimation, plasticity and high degree of disorder.

The widely used rigid molecule assumption is discussed in the light of Raman scattering experiments performed on orientationally disordered crystals. Environment effects inducing the deformation of molecules during their reorientations can be observed by choosing appropriate experimental conditions. Examples are given in the case of adamantane single crystals and neopentane.

The low-temperature laser- Raman and infrared absorption spectra of an oriented single crystal of orthorhombic sulfur have been recorded. The assignment of the ν 1 , ν 5 and ν 7 modes (all at about 470 cm −1 ) is made; the frequency of the ν 3 mode, inactive in the molecule, but active in the crystal, is confirmed. The presence of only one b 1 u band of low frequency leads to the assignment of 30 and 45 cm −1 for the librations. Thus, translations are in the 50–70 cm −1 range.

Polarized (VV) and depolarized (VH) Raman scattering have been performed on the 922 cm−1 internal line of neopentane both in the liquid phase and a multidomain single crystal of the plastic phase between 250 and 210 K. Small but systematic differences between the VV and VH spectra of the plastic phase, which are absent in the liquid phase, show anisotropy effects unrevealed up to now in this phase. The origin of the effect is discussed.

The polarized Raman spectra of monoclinic CBr4 are interpreted in terms of pseudo-cubic site symmetry 42m. The large polarizability of Br atoms is responsible for the high intensity of the off-diagonal terms of total symmetric tensors. Dipole–dipole coupling gives, for the v3 mode, a quite different behaviour from that of the other internal modes.

High and low frequency Raman scattering experiments have been performed in the whole temperature range of plastic neopentane (140–257 K). High frequency spectra are well accounted for by Larsson’s isotropic two‐step stochastic model when the vibrational contribution is not neglected. The discussion shows that the results are the best for the spectral part where the isotropic aspect of the model is less important. The low frequency spectra are explained by a transient dielectric anisotropy of the molecules induced by their deformation as they overcome a high potential barrier during reorientation movements. The analysis of both spectra provide consistent results for the temperature dependence of the fraction of molecules performing large amplitude rotations.

We report evidence for the existence of a new incommensurate phase in a crystal of ammonium hydrogen oxalate hemihydrate. This phase is remarkable in two aspects: it exists only above a critical pressure Pc, and the incommensurate wave vector, which is parallel to the vector c* of the reciprocal lattice, has the largest variation ever reported, varying continuously from 0.147c* at 4.3 kbar to ~ 0.25c* at the maximum pressure (8 kbar) used to date.

Infrared and Raman spectra have been recorded for the following dinuclear ions and molecules containing quadruple metal-to-metal bonds: Re2Cl8 2-, Re2Br8 2-, Mo2Cl8 4-, Re2(O2CCH3)Cl2, Re2(O2CCH3)4Br2 and Mo2(O2CCH3)4. The Raman data are, regrettably, rather incomplete due to the intense and extensive electronic absorption spectra of most of the compounds. Intense Raman lines observed at 275 and 289 cm−1 for the rhenium compounds and at 406 cm−1 for Mo2(O2CCH3)4 have been assigned as predominantly M-M stretching modes. Vibrational analyses using both valence force fields with a few interaction constants and Urey-Bradley force fields lead to moderately consistent results, with the M-M force constants lying in the range 3.1–4.5 mdyne/ A.

Resume The vibrational spectra of 31 crystalline hexachloro and hexabromo-metallates have been recorded. The calculation of force constants has been carried out from a modified Urey-Bradley force field, including the effect of cations on the internal vibrations of the anion. The relation between the degree of covalency of the metal-halogen bond and the stretching force constant is discussed.

High and low frequency Raman scattering experiments have been performed in the whole temperature range of plastic neopentane (140–257 K). High frequency spectra are well accounted for by Larsson’s isotropic two‐step stochastic model when the vibrational contribution is not neglected. The discussion shows that the results are the best for the spectral part where the isotropic aspect of the model is less important. The low frequency spectra are explained by a transient dielectric anisotropy of the molecules induced by their deformation as they overcome a high potential barrier during reorientation movements. The analysis of both spectra provide consistent results for the temperature dependence of the fraction of molecules performing large amplitude rotations.

Le spectre Raman d'un monocristal de sulfate de potassium K2SO 4 a été enregistré, à la température ordinaire, avec un spectrophotomètre équipé d'un laser à hélium-néon. Les bandes sont attribuées sans ambiguïté aux types de vibration Ag, B1g, B2g et B3g. L'influence de l'entourage des ions sulfate sur leurs modes internes de vibration est mise en évidence, ainsi que le mélange des modes de libration et de translation.

Resume The infrared spectra of some alkaline hexahalometal complexes have been recorded in the region 400–20 cm −1 , as crystalline powders.