Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Far-infrared spectrum and dielectric function of KCN and NaCN in the disordered cubic phase

The far-infrared reflection spectrum of KCN and NaCN in their cubic disordered phases is measured between 20 and 400 cm^?1 with dispersive Fourier-spectroscopy and the complex dielectric function ε̂(ω) is evaluated. At 295 K KCN exhibits a well defined TO mode at 148 cm^?1. The LO mode is at 234 cm^?1. The dielectric function of NaCN between 295 and 363 K is more complicated. Below 150 cm^?1 it can be approximated by an overdamped oscillator at 120 cm^?1.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Observation of a soft phonon in linear chain compound (NbSe4)3I by Raman scattering

The phase transition of the linear chain compound (NbSe₄)₃I was studied by Raman scattering. At 78 K three new peaks were observed at 73 cm^?1, 205 cm^?1 and 261 cm^?1. The totally symmetric Raman peak at 73 cm^?1 shows anomalous temperature dependence. The frequency decreases with increasing temperature, and at high temperatures an anticrossing occurs with another peak observed at about 58 cm^?1. The Raman intensity decreases and the linewidth broadens remarkably as the temperature increases. These properties allow us to assign this peak to a soft phonon. This fact indicates clearly the existence of a structural phase transition of a displacive type below room temperature.     

The adsorption of ammonia on a Fe(110) single crystal surface studied by high resolution electron energy loss spectroscopy (EELS)

EELS spectra of ammonia adsorbed on a Fe(110) single crystal surface at 120 K reveal four different molecular adsorption states:1. At very low exposures (0.05 L) three vibrational losses at 345 cm^?1, 1170 and 3310 cm^?1 are observed which are attributed to the symmetric Fe-N stretching-, N-H₃ deformation and N-H₃ stretching modes of chemisorbed molecular ammonia, respectively. The observation of only three vibrational losses indicates an adsorption complex of high symmetry (C_3v).2. Further exposures up to 0.5 L cause the appearance of additional losses at 1450 cm^?1, 1640 cm^?1 and 3370 cm^?1. The latter two are interpreted as the degenerate NH₃ deformation and - stretching modes of molecularly adsorbed NH₃. The 1450 cm^?1 loss is a combination of the losses at 345 cm^?1 and 1105 cm^?1. The observation of 5 vibrational losses is consistent with an adsorption complex of C_s symmetry.3. In the exposure range from 0.5 to 2 L adsorption of molecular ammonia in a second layer is observed. This phase is characterized by a symmetric deformation mode at 1190 cm^?1 and by two additional very intense modes at 160 cm^?1 and 350 cm^?1 which are due to rotational and translational modes.4. Exposures above 2 L cause multilayer condensation of ammonia characterized by translational and rotational bands at 190 cm^?1, 415 cm^?1 and 520 cm^?1, and a symmetric deformation mode at 1090 cm^?1. A broad loss feature around 3300 cm^?1 is attributed to hydrogen bonding in the condensed layer.Thermal processing of a Fe(110) surface ammonia covered at 120 K leads to decomposition of the ammonia into hydrogen and nitrogen above 260 K. No vibrational modes due to adsorbed NH or HN₂ species were detected.     

Raman scattering from the molecular charge transfer crystal anthracene-PMDA

The Raman spectrum of the mixed stack charge transfer (CT) crystal anthracene-PMDA, has been measured in the wavelength region near the lowest CT transition at 5458 Å. Six low frequency modes are observed below 140 cm^?1, the mode at 130 cm^?1 showing strong resonance enhancement. These modes are assigned as librational motion of the anthracene and PMDA molecules by rigid body analysis of X-ray structural data.     

High-pressure Raman and infrared spectroscopic studies of ZrP2O7

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP₂O₇ to 23.2 and 13 GPa respectively. In the pressure range 0.7–4.3 GPa the lattice mode at 248 cm^?1 disappears, new modes appear around 380 and 1111 cm^?1 and the strong symmetric stretching mode at 476 cm^?1 softens, possibly indicating a subtle phase transition. Above 8 GPa all the modes broaden, and all of the Raman modes disappear beyond 18 GPa. On decompression from the highest pressure, 23.2, to 0 GPa all of the modes reappear but with larger full width at half maximum. Lattice dynamics of the high temperature phase of ZrP₂O₇ were studied using first principles method and compared with experimental values.     

Vibrational spectroscopic characterization of the magnesium borate-phosphate mineral lüneburgite

ABSTRACT

Lüneburgite, a rare magnesium borate-phosphate mineral from Mejillones, Chile, has been characterized using Raman and mid-infrared spectroscopy methods. Boron tetrahedra are characterized by sharp Raman band at 877?cm^?1, attributed to the ν₁[BO₄]^5? symmetric stretching mode. The phosphate anion is associated with a distinct band at 1032?cm^?1, attributed to the ν₃[PO₄]^3? antisymmetric stretching mode. The most intensive Raman band at 734?cm^?1 is ascribed to stretching vibrations of bridging oxygen atoms in boron–oxygen–phosphor bridges. Bonds associated with water bending mode and stretching vibration are observed at 1661?cm^?1 (infrared) and in the 3000–3500?cm^?1 region (Raman and infrared spectrum).     

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm^?1 and a new peak around 166 cm^?1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm^?1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (T_c?T)^{$\text{1}\text{2}$}. This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.     

Synthesis of Ge-Sn at high pressure and high temperature in a laser-heated diamond anvil cell

Ge–Sn compound is predicted to be a direct band gap semiconductor with a tunable band gap. However, the bulk synthesis of this material by conventional methods at ambient pressure is unsuccessful due to the poor solubility of Sn in Ge. We report the successful synthesis of Ge–Sn in a laser-heated diamond anvil cell (LHDAC) at ~7.6 GPa &; ~2000 K. In situ Raman spectroscopy of the sample showed, apart from the characteristic Raman modes of Ge TO (Г) and β-Sn TO (Г), two additional Raman modes at ~225 cm^?1 (named Ge–Sn₁) and ~133 cm^?1 (named Ge–Sn₂). When the sample was quenched, the Ge–Sn₁ mode remained stable at ~215 cm^?1, whereas the Ge–Sn₂ mode had diminished in intensity. Comparing the Ge–Sn Raman mode at ~225 cm^?1 with the one observed in thin film studies, we interpret that the observed phonon mode may be formed due to Sn-rich Ge–Sn system. The additional Raman mode seen at ~133 cm^?1 suggested the formation of low symmetry phase under high P–T conditions. The results are compared with Ge–Si binary system.     

Quasi-symmetric top molecule approach to the rotational-vibrational problem of CH3XY molecules: Application to CH3OD

The quasi-symmetric top molecule approach proposed previously [J. Koput, J. Mol. Spectrosc.104, 12–24 (1984)] to calculate vibration-rotation energy levels of a CH₃XY molecule is used to study the COD bending-torsion-rotation energy levels of monodeuterated methanol, CH₃OD. The available 150 transition frequencies (microwave, millimeter wave, and infrared data) have been fitted and the barrier to linearity of the COD skeleton has been found to be about 7000 cm^?1. The effective barrier to internal rotation in the ground state has been determined to be 365.79 cm^?1 and that in the first excited state of the COD bending mode has been predicted to be 380.62 cm^?1.     

Study of lattice vibrational modes in CuGaS2 by using fourier transform spectroscopy

The lattice dynamics of a single crystal of CuGaS₂, grown by iodine transport technique, have been studied by using far IR absorption spectroscopy. All the absorption maxima caused by the phonon excitation are compared with the lattice vibrational modes obtained by Raman spectroscopy and by IR reflection techniques. An absorption maximum located at 175 cm^?1 cannot be explained with the help of phonon excitation; however this peak can be attributed to the defect frequency originating from the replacement of gallium atom by sulphur in the v₁₇ mode of vibration. The frequency of this defect-induced vibrational mode is calculated by taking a modified molecular model approach, and is found to be 166.9 cm^?1, which is in reasonably good agreement with the experimentally observed value of 175 cm^?1.     

Infrared study of superlattice formation in Ti1+xSe2

Infrared reflectivity measurements have been made on two Ti_1+xSe₂ crystals of different stoichiometry over the range 200 cm^?1 to 4000 cm^?1 before and after the formation of a 2a_o × 2c_o superlattice. In both crystals at room temperature a heavily damped Drude edge is observed at about 1000 cm^?1. At liquid helium temperatures, below the phase transition, the Drude minimum of a non-stoichiometric crystal ($\text{x ? 0.02, T}{}_{\mathrm{c}}\text{= 155 K}$) is at 750 cm^?1 whereas the low temperature minimum of a more stoichiometric sample (T_c = 198 K) is below 350 cm^?1. Also, new absorption peaks are seen to develop below T_c at about 3700 cm^?1 and 3300 cm^?1 for the non- stoichiometric and stoichiometric-crystals respectively. The observed phenomena are related to the energy gaps which open up in the band structure below the phase transition temperature.     

Electronic Raman spectra of iron doped MgO

The nearby excited states of Fe²⁺ in MgO are investigated by Raman spectroscopy. We observed an A_1g impurity mode (185 cm^?1) and an electronic transition at 110.5 ± 0.8 cm^?1 which we associate with the first excited states of the ferrous ion, Γ₃ and Γ₄, previously observed by far infrared optical absorption.     

Low frequency Raman mode in smectic liquid crystals near the phase transitions

A low-frequency intermolecular Raman mode at ～ 24 cm^?1 was observed in the solid phase of two related smectic A compounds. This mode shifts abruptly to lower frequency and decreases in intensity at the transition to smectic, while vanishing in the liquid phase. Qualitative interpretation of the results is given.     

Raman scattering study of the dynamics of the pressure-induced phase transition in thallium azide (TlN3)

A Raman scattering investigation of the pressure-induced phase transition in tetragonal thallium azide (TlN₃) is reported. The most interesting features of the Raman spectrum of TlN₃ are the anti-resonant line-shapes of two E_g symmetry phonons at 35 and 50 cm^?1 superimposed on a quasi-elastic wing. The scattering data is shown to be consistent with a model in which the two phonons interact via an imaginary coupling term. The phonon at 35 cm^?1 (assigned to a translational shear mode of the Tl⁺ sublattice) softens with increasing pressure and increases in linewidth as P approaches P₀ (=8 kbar) from below. At the same time, the quasi-elastic scattering component (associated with large amplitude N₃^? librational fluctuations) becomes less damped. A displacive structural transition from tetragonal to monoclinic is indicated by the eigenvector of the soft phonon.     

A far-infrared bolometric observation of the pinned charge-density wave in TTF-TCNQ

The polarized bolometric signal between 5 and 50 cm^?1 from a TTF-TCNQ single crystal near 10 K has been successfully fitted to a Lorentzian oscillator of natural frequency 3.4 ± 0.6 cm^?1 and a damping of 0.05 ± 0.03 cm^?1. This is presumably the phase mode of the pinned charge-density wave, with interchain Coulomb interaction probably being responsible for the pinning. The low damping is thought to be due to two-phonon “difference” processes, which are also thought to be responsible for a temperature-dependent feature at 25 cm^?1. No dependence of the pinned-mode frequency on electric field could be detected.     

Comparative Fourier Transform Infrared Investigation of Oltu-Stone (Natural Carbon Black) and Jet

Fourier transform infrared (FT-IR) investigation of Oltu-stone (natural carbon black) and jet revealed several differences between these carbonaceous materials. The band peaking at about 1000 cm^?1 is the first important difference: while the band in the jet spectra appears as one sharp peak at about 1001 cm^?1, the similar band in the Oltu-stone spectra is shifted to about 1026 cm^?1 with a broad shoulder toward high frequency. Even though the assignment of the shifted band is at present controversial, it may be attributed to carbon-oxygen stretching mode. Second, the doublet bands at about 2912 and 2843 cm^?1 are much more intense in the jet spectra then in the Oltu-stone spectra. They are confidently attributed to aliphatic C-H stretching mode. Finally, the broad water band on setting at about 3750 cm^?1 is maturated in Oltu-stone, and it is much more evident than in that of jet.

Therefore, FT-IR appears as a favorable identification method for these kinds of carbonaceous materials.     

Impurity induced far infrared absorption in doped NaCl and KCl

Far infrared absorption measurements with NaCl single crystals doped with AgCl and CuCl respectively show resonant band mode lines at 4.2 ?K at 52.5 cm^?1 and 23.6 cm^?1 respectively. Their measured halfwidths are 10 cm^?1 and 0.7 cm^?1 respectively. The halfwidth of the NaCl: Cu⁺ line increases strongly with increasing temperature. In NaCl:Mg⁺⁺, NaCl:Ca⁺⁺ and NaCl:Ag⁺ a temperature independent broad absorption is observed above 50 cm^?1, which behaves roughly as the density of phonon states in the unperturbed crystal. In KCl: Cu⁺ and KCl: Br^? a temperature independent absorption with a maximum at 108 cm^?1 is obtained corresponding to an expected region of high density.     

Four-photon laser spectroscopy of liquid water and aqueous solutions of α-chymotrypsin in the range ±5 cm<Superscript>−1</Superscript>

The resonances located at 1.2, 1.6, 2.0, and 2.3 cm^?1 with a width of ～0.2 cm^?1 were observed for the first time in the range ±5 cm^?1 of the four-photon Rayleigh wing spectra of distilled water and aqueous solutions of the protein α-chymotrypsin. The line at 2.3 cm^?1 belongs to the rotational transition 3(2, 1)–4(1, 4) of the ground vibrational state of water. In the presence of the protein, the spectrum is modified by the appearance of new lines, located at 0.74, 2.8, and 3.2 cm^?1. The modification of the spectrum observed is interpreted as a manifestation of low-frequency vibrations of large molecular fragments in aqueous protein solutions and as a result of the structuring of water in the vicinity of protein molecules.