Low-frequency Raman Spectroscopy Research Articles

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

Short-time dynamics of ionic liquids has been investigated by low-frequency Raman spectroscopy (4 < ω < 100 cm(-1)) within the supercooled liquid range. Raman spectra are reported for ionic liquids with the same anion, bis(trifluoromethylsulfonyl)imide, and different cations: 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-butyl-1-methylpiperidinium, trimethylbutylammonium, and tributylmethylammonium. It is shown that low-frequency Raman spectroscopy provides similar results as optical Kerr effect (OKE) spectroscopy, which has been used to study intermolecular vibrations in ionic liquids. The comparison of ionic liquids containing aromatic and non-aromatic cations identifies the characteristic feature in Raman spectra usually assigned to librational motion of the imidazolium ring. The strength of the fast relaxations (quasi-elastic scattering, QES) and the intermolecular vibrational contribution (boson peak) of ionic liquids with non-aromatic cations are significantly lower than imidazolium ionic liquids. A correlation length assigned to the boson peak vibrations was estimated from the frequency of the maximum of the boson peak and experimental data of sound velocity. The correlation length related to the boson peak (∼19 Å) does not change with the length of the alkyl chain in imidazolium cations, in contrast to the position of the first-sharp diffraction peak observed in neutron and X-ray scattering measurements of ionic liquids. The rate of change of the QES intensity in the supercooled liquid range is compared with data of excess entropy, free volume, and mean-squared displacement recently reported for ionic liquids. The temperature dependence of the QES intensity in ionic liquids illustrates relationships between short-time dynamics and long-time structural relaxation that have been proposed for glass-forming liquids.

Orientation of spheroid-shaped silver nanoparticles in mesostructured silica films studied by polarized absorption and low-frequency Raman spectroscopy

Orientation of spheroid-shaped silver (Ag) nanoparticles (NPs) embedded in mesostructured silica sol gel films is studied by polarized absorption measurements and low-frequency Raman scattering, two complementary techniques well-adapted to characterize nanostructures in amorphous matrix. Comparison is made with analysis of mesostructured silica sol gel films containing nanospheres. Polarization-dependent absorption spectra clearly evidence spheroid-shaped NP orientation and their mean aspect ratio is estimated from corresponding polar plots. Mean diameter of spherical NPs is deduced from low-frequency Raman scattering spectra as well as size of elongated spheroid NPs.

Spherical vibrational modes of ZrO 2–CuO nanoparticles

The nano-structural changes in zirconia (ZrO 2) ceramics upon incorporation of different amounts of the Cu 2+ cations and after thermal treatment of the amorphous precursors were investigated by low frequency Raman spectroscopy (LFRS). The spherical vibrational modes were observed for both – amorphous and nanocrystalline ZrO 2–CuO (ZC) particles. The procedure how to determine the particle size distribution from the LFRS modes was described and tested on few ZC samples by comparison with transmission electron microscopy results. The method was used to monitor the particle size growth in dependence on dopant concentration and annealing temperatures. The reduction of the particle size growth with doping, as well as the discontinuity in the particle size growth between 500 °C and 600 °C was observed. The first effect was interpreted with an increase of defects in nanoparticles with doping, and the second one by the phase transition from tetragonal to monoclinic phase of ZC particles.

The contribution of Raman spectroscopy to the analysis of phase transformations in pharmaceutical compounds

We show in this paper the contribution of the whole Raman spectrum including the phonon spectrum, to detect, identify and characterize polymorphic forms of molecular compounds, and study their stability and transformation. Obtaining these kinds of information is important in the area of pharmaceutical compounds. Two different polymorphic systems are analyzed through investigations in indomethacin and caffeine exposed to variable environmental conditions and various stresses, as possibly throughout the production cycle of the active pharmaceutical ingredient. It is shown the capability of the low-frequency Raman spectroscopy to reveal disorder in the crystalline state, to detect small amorphous or crystalline material, and to elucidate ambiguous polymorphic or polyamorphic situations.

Measurements of fractal dimensionality in 0.1Cs 2O–0.9TeO 2 glass-forming tellurite system by Raman spectroscopy

Low-frequency Raman spectroscopy was utilized in order to estimate the fractal dimensionality for tellurite 0.1Cs 2O–0.9TeO 2 binary glass-forming system in a temperature range, which includes the glassy and supercooled liquid state. A variation in fractal dimensionality was observed and calculated directly from the Raman spectroscopic data. This variation can be interpreted as an indication of structural alterations caused by temperature variation and correlated with existing structural models concerning tellurite network. Low-frequency Raman spectroscopy has proved to be a valuable tool for studying the fractal dimensionality and structural transformation in amorphous phases. The estimation of the localization degree of lattice vibration in disordered materials is important and discussed in the context of current phenomenological status of the field.

Study of polyethylene structure by means of low-frequency Raman spectroscopy: Longitudinal acoustic mode and boson peak

The opportunities given by the low-frequency Raman spectroscopy for studying the ordered structures in polymers are presented and illustrated by examples of application of two techniques. The longitudinal acoustic modes (LAM) excited in straight chain segments of macromolecules are characteristic in linear semicrystalline polymers, while the Boson peak (BP) manifests itself in amorphous (glassy) polymers. Both these features are due to the laser light scattering from the acoustic vibrations localized either in the ordered one-dimension structures (LAM) or in the mid-range order correlation sphere (BP).

Low Frequency Spectrumof Ionic Liquid [bmim][PF&lt;sub&gt;6&lt;/sub&gt;] Studied by Femtosecond Optically Heterodyne-Detected Optical Kerr Effect Spectroscopy and Low Frequency Raman Spectroscopy

利用飞秒光学外差光学克尔效应技术获得了咪唑类离子液体1-丁基-3-甲基咪唑六氟磷酸盐（[bmim][PF6]）体系中反映分子间相互作用的低频光谱.结果分析中,利用Brownian振子时间相关函数的精确表达式对光学克尔效应动力学数据中的核振动部分进行模拟,在时域上获得各个振动组分的时间演化行为,并进而通过快速Fourier变换获得各个振动组分的频域光谱和总加和光谱.此外还利用低频拉曼光谱技术测量了离子液体[bmim][PF6]的低频光谱,并和飞秒光学外差光学克尔效应实验的结果进行对比,发现两种方法获得的低频光谱相近.实验结果表明Brownian振子时间相关函数的精确表达式可用来解析离子液体体系中低频振动的时间演化行为.

Low‐Frequency Raman Scattering from Nanocrystals Caused by Coherent Excitation of Phonons

Since the first observation of surface acoustic modes from silicon nanocrystals (NCs) embedded in silica byDuval et al., low-frequency Raman scattering from NCs has become an important researcharea inmanyfields including semiconductor technologies, medical therapeutics, and biophysics. Recent research has indicated that low-frequency Raman spectroscopy is a feasible non-destructive technique for investigating virus functionalization, for example, introducing viruses on different materials, attaching viruses to quantum dots and carbon nanotubes, and forming multiple superstructures. These superstructures are expected to have important applications in biological science and medicine. However, the current assignment of the low-frequency Raman modes is based on Lamb’s theory that mainly focuses on the mode frequencies of an elastic vibration of a free isotropic sphere. Many studies have demonstrated that the surface acoustic-mode frequencies observed from free NCs and NCembedded matrix systems are consistent with the theoretical prediction. This is understandable because a large number of frequency values can be selected to match the experimental results. At the same time, other studies have unequivocally

Using the low-frequency Raman spectroscopy to analyze the crystallization of amorphous indomethacin

This paper gives a detailed analysis of the low-frequency Raman spectrum (LFRS) in the 5–250 cm −1 region, corresponding to collective vibrations, in the crystalline forms and in the amorphous state of indomethacin (IMC). This study points out the high sensitivity of the LFRS to detect, identify and evaluate the first traces of crystallization in comparison with high-frequency regions where internal vibration bands are detected. This analysis reveals that amorphous IMC prepared by cryogrinding instantaneously partially crystallizes at room temperature in the stable γ phase, well below T g = 43 °C. A method based on the treatment of the LFRS to determine precise and very low volume of crystallized material within amorphous matrix is described and used to analyze the crystallization kinetics of ground amorphous IMC powder. This study demonstrates that Raman spectroscopy is also a well-adapted technique to point out small amount of amorphous state in crystalline matrix. Crystallization of ground IMC powder was also analyzed by isothermal microcalorimetry experiments, which is one of the most widely used methods to analyze isothermal crystallization and to evaluate crystallinity.

Low-Frequency Raman Spectroscopy of Pure and La-Doped TiO2Nanopowders Synthesized by Sol-Gel Method

morphological characteristics of nanopowders on synthesis conditions and La 3+ content is investigated by the Raman spectroscopy. Very intensive modes observed in the Raman spectra of all nanopowder samples are assigned to anatase phase of TiO2. Additional Raman modes of extremely low intensity can be related to the presence of a small amount of brookite amorphous phase in nanopowders, which is in accordance with the results of X-ray difiraction analysis. The particle size distribution in TiO2 nanopowders was estimated from the low frequency Raman spectra, using the fact that the phonon modes in nanosized TiO2 observed in the low frequency region (! < 40 cm i1 ) can be well described by the elastic continuum model, assuming that nanoparticles are of perfect spherical shape and isotropic. The nanosized particle distribution obtained by this method is used for the calculation of the frequency and shape of the most intensive Eg Raman mode in anatase TiO2 by the phonon conflnement model. The calculated broadening of this mode, associated with the particle size distribution, coincides well with the characteristics of Eg mode observed in measured Raman spectra of TiO2 nanopowders. This conflrms the Raman spectroscopy method as a powerful tool for determination of particle size distribution in nanosized materials.

Remote Structural Correlation in the Sequence Reactor Powder–Gel–Fiber in Polyethylenes

A series of gels originated from ultra-high-molecular-weight polyethylene reactor powders, differing in morphology due to their different synthesis prehistory, were utilized for obtaining oriented fibers through the gel technology. The source powders, gels, and drawn fibers were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and low-frequency Raman spectroscopy. A comparison of the SEM images of powders with their straight chain segment (SCS) length distributions derived from the Raman spectra showed that the samples with pronounced fibrous morphology exhibited bimodal distribution functions, whereas the granular morphological pattern was specified by the unimodal SCS length distribution. Some remnant features of the ordered structure inherent in the powders were revealed in the gels. The drawability of gel-derived fibers was found to be dependent on the SCS length distributions in the gels and, indirectly, on the morphology of the reactor powders.

Shielding of Ionic Interactions by Sulfur Dioxide in an Ionic Liquid

The effect of adding SO2 on the structure and dynamics of 1-butyl-3-methylimidazolium bromide (BMIBr) was investigated by low-frequency Raman spectroscopy and molecular dynamics (MD) simulations. The MD simulations indicate that the long-range structure of neat BMIBr is disrupted resulting in a liquid with relatively low viscosity and high conductivity, but strong correlation of ionic motion persists in the BMIBr-SO2 mixture due to ionic pairing. Raman spectra within the 5<omega<200 cm(-1) range at low temperature reveal the short-time dynamics, which is consistent with the vibrational density of states calculated by MD simulations. Several time correlation functions calculated by MD simulations give further insights on the structural relaxation of BMIBr-SO2.

Investigation of the fast relaxation in glass-forming selenium by low-frequency Raman spectroscopy

The low-frequency Raman spectra of liquid and vitreous selenium are investigated. It is demonstrated that the temperature dependence of the intensity of the fast relaxation at the glass transition temperature (Tg = 308 K) exhibits a specific feature. This feature manifests itself in a sharper increase in the intensity at temperatures T > Tg as compared to that observed at lower temperatures. The intensities of the fast relaxation at the critical temperature Tc are evaluated by the extrapolation of the linear dependence to the temperature range T > Tg in the framework of the mode-coupling theory. The new results obtained for selenium are compared with the available data for other glass-forming materials (boron oxide, toluene, arsenic sulfide). It is shown that, for all the glasses under investigation, the parameter describing the contribution of the fast relaxation to the Raman spectrum takes on the same value at the critical temperature Tc and is approximately equal to 0.3.

Hydrogen bond dynamics and water structure in glucose-water solutions by depolarized Rayleigh scattering and low-frequency Raman spectroscopy

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.

Effect of Pb on the low frequency Raman modes in Bi2−xPbxSr2CaCu2O8+δ and Bi2−xPbxSr2Ca2Cu3O10+δ superconductors

Low frequency Raman spectroscopy measurements have been made on Bi2−xPbxSr2CaCu2O8+δ and Bi2−xPbxSr2Ca2Cu3O10+δ high temperature superconductors. The frequency of the ∼118cm−1 peak is found to correlate with the Pb content, and there is no correlation with the hole concentration in the CuO2 planes when it is varied by changing the oxygen content in the BiO layers. Thus, Raman measurements provide a means by which the Pb content can be estimated. When used in conjunction with measurements of the frequency of the O(2)SrA1g Raman mode, it is possible to also estimate the hole concentration in the CuO2 plane.

Straight-Chain Segment Length Distributions in UHMWPE Reactor Powders of Different Morphological Types

A series of reactor powders of ultrahigh molecular weight polyethylene obtained using supported and unsupported Ziegler catalysts were studied with the help of low-frequency Raman spectroscopy. This experimental technique allows one to calculate the length distribution of straight-chain segments (SCS) in a polymer sample without differing between the SCS localized in either crystalline or amorphous regions of the sample. A comparison of the scanning electron microscopy images of powders with their SCS distributions showed that the samples with pronounced fibrous morphology exhibit bimodal distribution functions, while the granular morphological pattern yields unimodal SCS length distributions.

Characterization of Temperature-Induced Phase Transitions in Five Polymorphic Forms of Sulfathiazole by Terahertz Pulsed Spectroscopy and Differential Scanning Calorimetry

The far-infrared properties of all five known polymorphic forms of the drug sulfathiazole have been studied by terahertz pulsed spectroscopy and low-frequency Raman spectroscopy. The observed spectra of the different polymorphs are distinctly different. Terahertz pulsed spectroscopy proves to be a rapid and complementary alternative to other physical characterization techniques reported in the literature for distinguishing between the five forms. Variable-temperature measurements (293–473 K) of all polymorphic forms have been performed. The phase transitions observed have been related to thermal analysis data. Form I is the form stable at high temperature of sulfathiazole with a melting point of about 475 K. Form II melts at around 470 K and recrystallizes at higher temperatures to form I. Forms III, IV, and V all convert to form I via a solid–solid phase transition at temperatures below 450 K. The phase transitions can be monitored by terahertz pulsed spectroscopy. Polymorphic impurities of the samples can be detected in the room temperature spectra and their effect on the phase transition behavior can be studied

Liquid Structure and Preferential Solvation of Metal Ions in Solvent Mixtures of N,N-Dimethylformamide and N-Methylformamide

Raman spectra of aprotic N,N-dimethylformamide (DMF) and protic N-methylformamide (NMF) mixtures containing manganese(II), nickel(II), and zinc(II) perchlorate were obtained, and the individual solvation numbers around the metal ions were determined over the whole range of solvent compositions. Variation profiles of the individual solvation numbers with solvent composition showed no significant difference among the metal systems examined. In all of these metal systems, no preferential solvation occurs in mixtures with DMF mole fraction of x(DMF) < 0.5, whereas DMF preferentially solvates the metal ions at x(DMF) > 0.5. The liquid structure of the mixtures was also studied by means of small-angle neutron scattering (SANS) and low-frequency Raman spectroscopy. SANS experiments demonstrate that DMF molecules do not appreciably self-aggregate in the mixtures over the whole range of solvent composition. Low-frequency Raman spectroscopy suggests that DMF molecules are extensively hydrogen-bonded with NMF in NMF-rich mixtures, whereas NMF molecules extensively self-aggregate in DMF-rich mixtures, although the liquid structure in neat NMF is partly ruptured. The bulk solvent structure in the mixtures thus varies with solvent composition, which plays a decisive role in developing the varying profiles of the individual solvation numbers of metal ions in the solvent mixtures.

Evidence of a two-stage thermal denaturation process in lysozyme: A Raman scattering and differential scanning calorimetry investigation

Raman spectroscopy (in the low-frequency range and the amide I band region) and modulated differential scanning calorimetry investigations have been used to analyze temperature-induced structural changes in lysozyme dissolved in 1H2O and 2H2O in the thermal denaturation process. Low-frequency Raman data reveal a change in tertiary structure without concomitant unfolding of the secondary structure. Calorimetric data show that this structural change is responsible for the configurational entropy change associated with the strong-to-fragile liquid transition and correspond to about 1/3 of the native-denaturated transition enthalpy. This is the first stage of the thermal denaturation which is a precursor of the secondary structure change and is determined to be strongly dependent on the stability of the hydrogen-bond network in water. Low-frequency Raman spectroscopy provides information on the flexibility of the tertiary structure (in the native state and the transient folding state) in relation to the fragility of the mixture. The unfolding of the secondary structure appears as a consequence of the change in the tertiary structure and independent of the solvent. Protein conformational stability is directly dependent on the stability of the native tertiary structure. The structural transformation of tertiary structure can be detected through the enhanced 1H/2H exchange inhibited in native proteins. Taking into account similar features reported in the literature observed for different proteins it can be considered that the two-stage transformation observed in lysozyme dissolved in water is a general mechanism for the thermal denaturation of proteins.

Association of rhodamine 6G in light and heavy water solutions

The absorption characteristics of rhodamine 6G molecules in light and heavy water solutions are studied. The association efficiencies of dye molecules and the structure and the binding energy of complexes formed are determined in relation to the solvent composition. The structure of H2O +D2O solutions is studied using correlation and low-frequency Raman spectroscopy. The distribution of dissolved complex molecules in the water matrix is shown to have inhomogeneities, which are responsible for a high association efficiency. The characteristic size of these inhomogeneities is estimated. The fractal dimension of the structural formations of light and heavy water molecules is determined as a function of their concentration.