Raman polarization analysis of highly crystalline polyethylene fiber

The complex orientation dependence in space of Raman active vibrations in the orthorhombic structure of polyethylene (PE) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. Building upon the symmetry assignment of these vibrational modes, we systematically studied, from both theoretical and experimental viewpoints, the changes of polarized intensity for the A_g and the B_2g + B_3g vibrational modes with respect to PE molecular orientation. After explicitly expanding the Raman selection rules associated with the A_g and the B_2g + B_3g modes, introducing them into general expressions of the orientation distribution function, and validating them by means of a least‐square fitting procedure on experimental data, we compare here two mesostructural models for a highly crystallized and self‐aligned PE fiber structure. Stereological arguments are shown concerning the arrangement of orthorhombic fibrils in such a sample that unfold the correct values of five independent Raman tensor elements for orthorhombic PE. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman and infrared reflection spectroscopy in black phosphorus

The symmetry and energies of all optically active phonon modes in black phosphorous are determined by polarized Raman scattering and infrared reflection spectroscopy at room temperature. The obtained energies are; 365 and 470 cm^-1 for A_g modes, 197 for B_lg, 442 for B_2g, 223 and 440 for B_3g, 136 (TO) and 138 (LO) for B_lu, and 468 (TO) and 470 (LO) for B_2u, respectively. The small TO-LO splitting is related to the charge transfer between phosphorus atoms induced by the atomic displacement.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Phonons in mixed crystals

A concept is presented which gives qualitative information concerning the phonon behaviour of ionic mixed crystalsAB _1–x C _x at certain symmetry points on the zone boundary and consequently through the zone to the center. The model assumes that the end-member phonons and the impurity-modes ofAB:C andAC:B are known and uses the assumption that mixed crystal phonons behave symmetry-wise as the corresponding end-member phonons. The concept is demonstrated for several cases of mixed crystals.     

Residual‐density mapping and site‐selective determination of anomalous scattering factors to examine the origin of the Fe K pre‐edge peak of magnetite

The electron‐density distribution and the contribution to anomalous scattering factors for Fe ions in magnetite have been analyzed by X‐ray resonant scattering at the pre‐edge of Fe K absorption. Synchrotron X‐ray experiments were carried out using a conventional four‐circle diffractometer in the right‐handed circular polarization. Difference‐Fourier synthesis was applied with a difference in structure factors measured on and off the pre‐edge (E_on = 7.1082 keV, E_off = 7.1051 keV). Electron‐density peaks due to X‐ray resonant scattering were clearly observed for both A and B sites. The real part of the anomalous scattering factor f′ has been determined site‐independently, based on the crystal‐structure refinements, to minimize the squared residuals at the Fe K pre‐edge. The f′ values obtained at E_on and E_off are ?7.063 and ?6.682 for the A site and ?6.971 and ?6.709 for the B site, which are significantly smaller than the values of ?6.206 and ?5.844, respectively, estimated from the Kramers–Kronig transform. The f′ values at E_on are reasonably smaller than those at E_off. Our results using a symmetry‐based consideration suggest that the origin of the pre‐edge peak is Fe ions occupying both A and B sites, where p–d mixing is needed with hybridized electrons of Fe in both sites overlapping the neighbouring O atoms.     

Vibrational modes symmetry and raman scattering in SnI4 crystals

A group-theoretical vibrational modes symmetry analysis of SnI₄ crystals has been made for the Brillouin zone center and polarized Raman scattering study in these crystals has been carried out. Correlation diagram showing the correspondence between the crystal vibrational modes and the free SnI₄ molecule modes has been built. The nature and peculiarities of the low frequencies spectrum have been considered.     

Surface‐enhanced Raman spectroscopy of indanthrone and flavanthrone

The normal Raman and surface‐enhanced Raman scattering (SERS) spectra of flavanthrone and indanthrone were obtained at several excitation wavelengths. The spectral assignments were aided by density functional calculations. Since both molecules have very high symmetry (C_2h) including a center of inversion, we expect that the modes of u symmetry will be forbidden in the normal Raman spectrum. However, proximity to the surface causes special SERS enhancement of several of the b_u modes, along with somewhat weaker enhancement of the a_u and b_g modes. Copyright © 2009 John Wiley & Sons, Ltd.     

利用偏振喇曼谱研究KBr:Pb2+单晶中的HD色心

本文报道掺铅溴化钾单晶在液氮温度下经大剂量X射线辐照后的偏振喇曼测量结果。利用几年前才发展起来的所谓“表现型”分析方法,确认喇曼谱中频移为186cm^-1的信号具有点群C_1k(010),它起因于H_D色心的A模振动。 关键词：     

Phase transitions in RbCaF3. I: Optical studies

Phase transitions in RbCaF₃ have been investigated by optical birefringence measurements and Raman scattering experiments. A near discontinuity in the onset of spontaneous birefringence at 196 K shows that the cubic to tetragonal phase change is “slightly” first order. The Raman spectra of the tetragonal phase support a D¹⁸_4h structure in which two phonons of A_1g and E_g symmetry soften as the 196 K transition is approached from lower temperatures. A very slow transition to a lower symmetry structure was observed at about 42 K in the Raman measurements.     

Zone edge phonons in CdS1−xSex

Zone edge phonons of mixed CdS_1?xSe_x have been studied by mean of infrared absorption and Raman scattering techniques. In the A point of the Brillouin zone, it has been shown that transverse acoustical phonons have a one mode behaviour, and that optical phonons have a two modes behaviour. CdS and CdSe zone center phonons can combined and give a LO(CdS) + LO(CdSe) Raman peak in addition to the 2LO(CdS) and 2LO(CdSe) peaks; this is not the case for phonons from the edge of the Brillouin-zone where no CdS + CdSe combination can take place.     

Raman spectra of fosterite

The normal modes of the Raman-active phonons in the center of the Brillouin zone in a natural monocrystal of Mg₂SiO₄ were measured by using Raman scattering. All of the modes predicted by group theory were observed and measured. Some deviations from previously published values in synthetic samples were observed for the low energy phonons.     

Temperature‐dependent low wavenumber Raman scattering studies in LiCsSO4 crystal

The Raman lattice modes in the LiCsSO₄ crystal were measured in the temperature range of 17–303 K. Two right‐angle scattering geometries were used: z(xx)y and z(xz)y, to observe A_g and B_g (or B_2g) modes, respectively. Critical phenomena were observed in the spectra at temperatures of about 200, 180 and 100 K. They correspond to structural phase transitions in the crystal from D_2h to C_2h at 200 K and to C_s symmetry at 100 K. The nature of the transition at 180 K is supposed to be of a lock‐in type, but it needs further investigation. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of the Resonance Raman Scattering Spectra and Line Spectra of Tetrabenzoporphin Metal Complexes

On the basis of the calculations of normal vibrations of Zn– and Cu–tetrabenzoporphin, the literature spectra of resonance Raman scattering of the above compounds and the fluorescence spectra of Zn–tetrabenzoporphin are interpreted. The nonmirror character of the fluorescence and fluorescence excitation spectra is noted for some B _1g symmetry type vibrations of the point group of symmetry D _4h caused by the symmetry breakdown of a molecule in its electronic state S ₁ due to the Jahn–Teller effect.     

Phonon spectra and lattice dynamics of lithium nitride

Lithium nitride, Li₃N, belongs to the hexagonal structure with the symmetry point group D_6h. The structure is ionic with four atoms per unit cell and N^3- coordinated in a regular way with eight Li⁺ ions. Polarized Raman scattering and infrared reflectivity spectra are reported. The frequencies of zone center phonons are determined from a Kramers-Kronig analysis and the oscillator fits to the reflectivity data. A rigid shell model with eight parameters, including the anisotropic polarizability of nitrogen ions, yields an excellent fit to the observed phonon frequencies.     

Vibrational Raman spectra of CdSx Se1‐x magic‐size nanocrystals

Resonant Raman scattering spectra of ultrasmall (<2 nm) magic‐size nanocrystals (NCs) are reported. The spectra of CdS and CdS_x Se_1‐x NCs, resonantly excited with 325 nm and 442 nm laser lines, correspondingly, reveal broad features in the range of bulk optical phonons. The relatively large width, ～50 cm^‐1, and downward shift, ～20 cm^‐1, of the Raman bands with respect to the longitudinal optical phonon in bulk crystals and large NCs are discussed based on the breaking of the translational symmetry and bond distortion in these ultrasmall NCs. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

自旋梯状化合物Sr14(Cu1-yFey)24O41的拉曼散射谱研究

利用常规的固相合成法制备了三种Sr₁₄(Cu_1-yFe_y)₂₄O₄₁(y=0,0.03,0.05)多晶样品,其中y=0.03是单纯Fe³⁺掺杂,y=0.05是Fe³⁺和Fe²⁺混合掺杂.在100—1500cm^-1频移范围内测量了这三种样品的偏 关键词： 拉曼散射 电输运性质 自旋梯状化合物     

Lattice dynamics of tetragonal R2BaCuO5 (R = La,Nd) oxides in the P4/mbm structure

A short‐range force constant model (SRFCM) has been applied to investigate the Raman and the infrared wavenumbers in R₂BaCuO₅ (R = La, Nd) in their tetragonal phase of space group P4/mbm. Calculations of zone‐center phonons are made with six stretching and five bending force constants. All the Raman and infrared values are then assigned to their corresponding modes. The force constants were evaluated by fitting ten Raman and eight infrared modes. The calculated Raman and infrared modes show good agreement with the observed values. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman scattering of NiS2

Raman scattering measurements of NiS₂ is done and five optical phonon peaks are found. These peaks are assigned to A_g, E_g and 3T_g phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS₂ and MnS₂.     

Raman active modes of NiSi crystal

Raman scattering intensities of the NiSi Raman-active modes have been calculated with three Raman measurement configurations, which can be used for the symmetry assignment of the NiSi Raman peaks. Raman-active vibrations of the NiSi crystal have also been theoretically studied. Results show that the lattices with A_g and B_2g modes vibrate only in the plane normal to the NiSi[0 1 0] direction while the lattices with B_1g and B_3g modes vibrate only along the NiSi[0 1 0] axis. Based on such study, the relationship between the anisotropic strain distribution in the NiSi thin film and the Raman peak shifts has been briefly discussed.     

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

In Dy³⁺ and Li⁺ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A₁ and E₁ symmetry also show a noticeable red shift from E₁ symmetry (in ab plane) to A₁ symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.