Low-frequency Raman Spectroscopy Research Articles

Vibrational Spectroscopic Study of Thiophenolate-Capped Nanoclusters of CdS and of Cadmium Thiophenolate Complexes

Thiophenolate-capped nanoclusters of cadmium sulfide and the cadmium thiophenolate compounds Cd(SPh)2, [Cd(SPh)4](Me4N)2, [Cd4(SPh)10](Me4N)2, and [Cd4X4(SPh)6](Et4N)2 (X = Cl, Br, I) have been studied by low-frequency FT-IR and Raman spectroscopy. In the spectrum of the cluster [S4Cd10(SPh)16](Me4N)4 a band at 288 cm-1 was assigned to the ν(Cd−S) stretching modes of the (Cd−S) bonds of the S4Cd6 cluster core. The ν(Cd−S) stretching frequencies of the bridging and terminal SPh- ligands which cap the cluster core were assigned to bands in the region 150−187 cm-1. The ν(Cd−S) core band for [S4Cd17(SPh)28](Me4N)2 with tetracoordinated sulfides, as in bulk CdS, was observed at 270 cm-1. The far-infrared ν(Cd−S) core band showed a progressive broadening and shift from 270 cm-1 toward the peak position of 241 cm-1 in bulk (wurtzite) CdS with increasing cluster size up to ca. 40 A. This shift is interpreted in terms of an increase in the average Cd−S bond length. Far-infrared spectroscopy has been shown to be a ...

Structural correlation and structural relaxation at the final stage of gel-to-glass transition in silica

In this work more detailed information is obtained about the final stage of the gel-to-glass transition and the interrelations between the glass network structure and the mechanical properties of the material are revealed. The structural modification of silica xerogels was followed by measuring Vickers hardness and Young's modulus in parallel with measurements of the network structure by means of IR reflection and low-frequency Raman spectroscopies. Two correlations of material properties and scales have been found: (i) at annealing temperature, T a=1000°C, the relaxation of the silica network produces a definite correlation sphere and an increase of Young's modulus to that for vitreous SiO 2; (ii) at T a=1160°C, a decrease of the mobility of glass-forming units causes a decrease of the correlation radius and an increase of Vickers hardness to that specific for vitreous SiO 2.

Temperature Dependence of Relaxation Time of Water in NaCl Aqueous Solution by Low-Frequency Raman Spectroscopy

Depolarized low-frequency Raman spectra of NaCl aqueous solution in the frequency region below 250 cm -1 have been investigated as a function of temperature. The spectral profiles have been analyzed with a superposition of one Cole-Cole type relaxation mode and two damped harmonic oscillator modes. The reciprocal relaxation time τ -1 changes linearly as a function of temperature below 320 K, which means that the relaxation time τ obeys Curie-Weiss law. The distribution parameter β in the Cole-Cole type relaxation formula decreases with decreasing temperature. With decreasing temperature the reciprocal relaxation time τ -1 goes down toward to zero more slowly than that in water, which means that the value of Curie constant of NaCl aqueous solution is larger than that of water.

Comparative study of size and distribution of lamellar thicknesses and long periods in polyethylene with a shish-kebab structure

This study contains a combined application of three different techniques for the study of injection moulded polyethylene (PE), showing an oriented shish-kebab structure: small angle X-ray scattering (SAXS), low frequency Raman spectroscopy (LAM) and transmission electron microscopy (TEM), A series of linear PEs and molecular weights in the range 51000–478000 has been investigated and two injection temperatures have been used (Tm=144 and 210 °C). SAXS patterns from the highly oriented regions show the presence of either one axial long period (L1) or two (L1 and L2) depending on molecular weight (¯Mw) and Tm. It is shown that L1 and L2 increase with ¯Mw up to a given critical molecular weight ¯Mc. Above ¯Mc, L1 and L2 remain constant. Raman results qualitatively confirm the existence of two separate distributions of straight-length chain segments for those samples having molecular weights above the critical value. Shorter segments are shown to be more abundant than the longer ones. In the lowest molecular weight sample, results from SAXS, TEM and Raman spectroscopy seem to be consistent with each other, although in some cases a tilted molecular arrangement within the lamellae has to be invoked. On the other hand, in case of the highest molecular weight sample, the length of the short straight-chain segments derived from Raman spectroscopy agrees well with the double periodicity obtained from SAXS. On the contrary, long periods measured from TEM only correspond to the shorter SAXS periodicity. This result is discussed by assuming the occurrence of crystalline bridges among adjacent lamellae.

Order in nascent polyethylene

Nascent polyethylenes of the high density and linear low density types prepared by gas phase polymerization, together with a series of nascent high density polyethylenes prepared by the slurry process, have been examined by X-ray diffraction, inelastic neutron scattering and Raman spectroscopy. The X-ray diffraction data and inelastic neutron scattering spectra show that the degree of crystallinity is smaller in the nascent samples than is commonly found in melt-crystallized materials. Low frequency Raman spectroscopy shows that the lamellar thickness is also smaller than usual. Melting and resolidification of the nascent, gas phase polymerized polyethylenes result in materials that more closely resemble the melt-crystallized products. The implications of the results are discussed as regards the crystallization process.

Use of isotope effects in studies of intermolecular interactions by Raman spectroscopy

A very brief introduction to vibrational spectroscopy is given with a particular emphasis on the effect of isotopic substitution. A few key references to studies of both the gaseous and the crystalline states are cited. Intermolecular interactions in liquids are studied by (i) Resonance Energy Transfer giving rise to a non-coincidence effect between the anisotropic and the isotropic Raman components, (ii) Coalescence of Bands in Mixtures of Isotopomers, giving rise to an observation of fewer bands than expected from a simple addition of bands from the individual isotopomers in the mixture, and finally (iii) low-frequency Raman spectroscopy, for which the R( v ̄ ) representation is used in order to get rid of the intense Rayleigh line appearing in the Raman spectra of liquids. Experimental data include results for isotopically substituted liquid formamides and a solution of HCOND 2 in D 2O. The results show that intermolecular interactions via transfer of vibrational energy occur for the carbonyl stretching vibration at around 1650 cm −1 (amide-I band). A band in the low-frequency spectrum at around 100 cm −1 is assigned to a mode involving displacements of atoms in the intermolecular hydrogen bonds. Similar interactions might be of importance for the fast dynamics of biological macromolecules like proteins and nucleic acids.

Low frequency Raman spectroscopy of supercooled fragile liquids analyzed with schematic mode coupling models

Recent experimental studies of the glass transition of molecular liquids have exploited light scattering techniques in order to support the dynamical model proposed by the mode coupling theory. In the framework of the dipole-induced-dipole (DID) formalism and the Stephen’s approximation, we have checked this theory with several memory functions in the microscopic region, where phononlike excitations dominate, i.e., in the frequency window of 5–130 cm−1 accessible by a classical Raman spectrometer. The fitting procedure compares the experimental susceptibility spectra of one of the simplest fragile molecular liquid, m-toluidine, to the theoretical ones and estimates, in each case, the T dependence of the different control parameters as well as the crossing point of the transition line of type B. The agreement observed for spectra from a temperatures above the melting point down to the glass transition temperature Tg suggests, on the one hand, that information about the dynamical behavior of the supercooled liquid are contained in this frequency region and, on the other hand, that vibrational contributions are incorporated in this formalism, independently of the form of the relaxation kernel. Finally, the two-peak shape in the microscopic range of the susceptibility spectra is related to the relaxation of a linear combination of the Fourier components of the two density correlators.

Low-frequency Raman spectroscopy

Some intensity expressions for low-frequency Raman, far-IR and inelastic neutron scattering are discussed. Low-frequency near infrared (NIR)-FT-Raman spectra (∼80 – 400 cm −1) of some proteins and their aqueous solutions are presented, along with low-frequency visible Raman spectra (10 – 400 cm −1) of their model systems, some liquid amides. A low-frequency band is observed in the R( \\ ̄ gn)-representation of these spectra at approximately 100 cm −1. This band is assigned to a mode involving atoms in hydrogen bonds. Three more quantitative models are used to describe it. The first model describes these low-frequency bands in terms of longitudinal and transverse mode density of states curves of one-dimensional hydrogen-bonded chains, the second model is a damped harmonic oscillator model, and the third model is the Gaussian cage model. A band with a maximum around 290 cm −1 might be sensitive to the water structure in aqueous protein solutions.

Humidity-dependent structural transition of guanosine and disodium adenosine 5'-triphosphate crystals studied by low-frequency Raman spectroscopy.

Humidity-dependent structural transition of guanosine and disodium adenosine 5'-triphosphate crystals studied by low-frequency Raman spectroscopy.

Low-frequency Raman spectroscopy of plastically deformed poly(methyl methacrylate)

The low-frequency Raman scattering (2–200 cm −1 range) of poly(methyl methacrylate) (PMMA) specimens plastically deformed in pure shear is compared to that of non-deformed specimens. The main experimental feature is an excess of Raman scattering in the 30–50 cm −1 spectral range induced by plastic deformation. This excess appears to be strongly anisotropic and is maximum when the light polarization is orthogonal to the principal tensile strain component, i.e. to the chain orientation direction. The effect is still present, but with a weaker anisotropy, after mechanical cycling when the macroscopic deformation is brought back to zero by application of an opposite stress. These observations are interpreted in terms of the non-continuous nature of the disordered network on the nanometric scale. The orientation of macromolecular strands induced by plastic deformation is shown to be higher in the less cohesive spaces.

Depolarized light scattering studies of the boson peak and the transversal phonons in a glass-forming liquid: OTP

A lineshape analysis of the Rayleigh wing of ortho-terphenyl obtained by low frequency Raman spectroscopy has been performed. The so-called boson peak was studied from 100 to 300 K, from far below to above the glass transition at T g = 244 K. A model of disorder-induced scattering was used to interpret the features of the boson peak. A complementary measurement of the transversal phonons of OTP was done in the temperature range from 250 to 360 K. It was found that the shear waves are only present up to 320 K.

Breakdown of the soft-mode model of KDP—low-frequency Raman spectroscopy under high pressure

Abstract The Raman scattering of KDP has been carried out under high pressure 0-5 GPa at two scattering angles, 90 and 4.5 degrees. The low frequency components of the B2(z) spectra in x(yx)y and z(yx)z+δ geometries have been analyzed in terms of a coupled soft-phonon model. The wave vector dependence of the soft-mode frequency contradicts the dispersion relation of the ferroelectric soft-mode near the Γ point of the Brillouin zone. The low frequency component of the B2(z) spectra is concluded not to be caused by the soft mode.

Raman and low frequency Raman spectroscopy of lead, zinc and barium metaphosphate glasses doped with Eu3+ ions

An investigation of low-frequency inelastic scattering (LOFIS) and pre-resonant Raman spectroscopy has revealed the role of the modifying cations, Ba2+, Zn2+ and Pb2+ on the bonding properties of the Eu-O bond in Ba(PO3)2, Zn(PO3)2, and Pb(PO3)2 glasses. Analysis of the results has shown that the electronegative character of the Pb2+ ions and the ionic radii of the Ba2+ and Zn2+ ions play a major role in the short-range order of the metaphosphate glasses. Furthermore, it was shown, using LOFIS, the influence of the modifying cations on the size distribution of the structural units found in the metaphosphate glasses.

Low-frequency Raman spectroscopy of deuterium to megabar pressures at 77-295 K.

Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have been measured from 1.8 GPa to [similar to]200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinuous changes in three distinct pressure ranges are observed. At lower pressures ([lt]30 GPa), there is a gradual increase in the linewidth of the [ital S][sub 0](0) band, together with gradually decreasing resolution of the higher-energy [ital S][sub 0](1) and [ital S][sub 0](2) features. At intermediate pressures (60--100 GPa), a change in the pressure dependence and linewidth of [ital S][sub 0](0), and a linewidth and intensity decrease in the [ital E][sub 2[ital g]] phonon band occur. This is interpreted as evidence for a phase transformation in the molecular solid beginning at [similar to]65 GPa (77 K). At the highest pressures ([similar to]160 GPa), abrupt changes in the low-frequency excitations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the low-frequency spectra for interaction between the deuterium sample and diamond anvil is also examined.

Various Scale Levels of the Gel-to-Glass Transformation

The gel-to-glass transition in SiO 2 xerogels prepared by inorganic sol-gel synthesis was studied. The evolution of the molecular structure is traced using the infrared and low-frequency Raman spectroscopy methods. The elastic moduli of the samples as well as the pore wall moduli at various stages of heat treatment are determined from the data on Brillouin scattering. The formation of monolithic glass on the macroscopic level manifests itself within a narrow temperature range by the dramatic increase of Young's modulus to the accepted value for fused silica. This phenomenon coincides with structural transformations on the molecular scale: (i) the definite correlation radius (the long-range order sphere) appears; (ii) local distortions of the silica network relax.

Structural characterization of Sol-Gel derived planar waveguides by very low frequency Raman spectroscopy

The sol-gel method has been used to prepare three different planar waveguides of low attenuation. The aim of the paper is to show that very low frequency waveguide Raman spectroscopy is useful for the characterization of the thin film.

Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy

Depolarized Raman spectra below 250 cm−1 in water and heavy water were measured and analyzed from 373 K to the supercooled region. The spectral feature of the central component below 20 cm−1 is stressed in the present work. The spectra below 250 cm−1 in water and heavy water are interpreted as a superposition of one Debye-type relaxation mode and two damped harmonic oscillators. The damped harmonic oscillators (broadbands around 60 cm−1 and 190 cm−1) are interpreted as the restricted translational modes. Analyzing the temperature dependence of the relaxation mode, the reciprocal relaxation time τ−1 in heavy water changes linearly with τ−1 ∝ (T − 240 K) in the whole temperature range. On the other hand, the temperature dependence of the reciprocal relaxation time in water above 303 K deviates from a straight line which holds below 298 K as τ−1 ∝ (T − 225 K).

Vibrational spectroscopic study of Ti-substituted SiO 2

The structure of SiO2 TiO2 glass has been studied at concentrations of titanium from 2.2 to 5.8 mol% by vibrational spectroscopy methods including low-frequency Raman spectroscopy. The glass samples were prepared by vapour deposition of silica and titanium tetrachlorides with concentration of TiCl4 in gas up to 8 vol.%. The temperature of deposition in the flame of hydrogen-oxygen torch was 1640°C. The Raman spectra were measured in a 90°-geometry using 488 nm light for excitation. The presence of Ti in vitreous silica is indicated in IR and Raman spectra by the appearance of a 935 cm−1 band whose intensity was found to increase monotonicly with increase of Ti content. When Ti is introduced, (i) the bond angle within bridging groups diminishes, (ii) the radius of structure correlation increases; and (iii) the concentration of intrinsic defects in the SiO2 matrix becomes two or three times smaller at Ti content of ∼4 mol%. With further increase of dopant concentration, it remains the same. Indication of clustering was not observed. Ti-substitution leads to an ordering of the structure of vitreous silica.

Study on Low-Frequency Raman Spectra of n-Decylammonium Chloride

The low-frequency Raman spectrum of n-decylammonium chloride was measured as a function of temperature in the temperature range from 290 to 340K, and the longitudinal acoustical mode vibration band was assigned. The results showed that there are two phase transitions at 313K and 321K, respectively. The phase transition at 313K is mainly induced by change of hydrocarbon chain conformations, while that at 321K is mainly induced by change of order degree of molecular packing. The results suggest low-frequency Raman spectroscopy is a useful probe of structural phase transition for long-chain compounds.

Crystallinity of uniform tri(oxyalkylene)s

The crystallinities of uniform with x = 3 to 6, were investigated by small-angle X-ray diffraction, low-frequency Raman spectroscopy, infra-red spectroscopy and differential scanning calorimetry. It is shown that the C4 to C6 trimers adopt a trans-planar conformation in their layer crystals, whereas the C3 trimer adopts a different chain conformation. In all cases the hydroxy end groups are extensively hydrogen bonded.