Weak Raman Bands Research Articles

Micro-Raman and infrared properties of SnO2 nanobelts synthesized from Sn and SiO2 powders

Rutile structured SnO2 nanobelts have been synthesized from the mixture of Sn powders and SiO2 nanoparticle powders. Each nanobelt has a uniform width of about several hundred nanometers and a thickness of about tens of nanometers along its entire length. Micro-Raman spectrum measurement on the SnO2 nanobelts shows that the first-order Raman A1g mode (632.9 cm−1) is very strong, and two weak Raman bands 498 and 694 cm−1 seem to correspond to infrared (IR)-active longitudinal optical (LO) and transverse optical (TO) of A2u modes. In addition, the IR spectrum of the SnO2 nanobelts shows the A2u (LO) (701.9 cm−1) and Eu (1) (TO) (634.5 cm−1) modes and one surface mode (565.2 cm−1). The IR-active bands in the Raman spectrum and the surface mode in IR spectrum, which may be due to the nanoscale morphology of the nanobelts.

How to determine the pressure of a methane‐containing gas mixture by means of two weak Raman bands, ν3 and 2ν2

AbstractRaman spectra of a pure CH4 sample, two CH4C2 H6 mixtures and a CH4N2 mixture were obtained as a function of pressure at pressures up to 39.6 MPaA (MPa absolute). These spectra are presented in the region 3120–2980 cm−1. A clear pressure dependence of the area ratio between two weak methane bands, ν3 (asymmetric CH stretching) and 2ν2 (overtone of the asymmetric CH bending), is observed; the methane ν3 band intensity is lowered relative to the methane 2ν2 as the pressure is raised. The intensity ratios between these two bands, I(ν3)/I(2ν2), were determined and plotted as a function of pressure. Surprisingly it is observed that the ratio at a fixed pressure is independent of the composition and thereby of the surroundings in which the methane molecule is vibrating. A model function to predict the pressure is given. From a practical point of view, the present results could be useful for determining directly the total pressure in methane mixtures the composition of which is not known. Copyright © 2002 John Wiley & Sons, Ltd.

Near-Infrared Raman Spectrometer Systems for Human Tissue Studies

We have built two types of instruments for near-infrared Raman spectroscopy studies of human tissues, one for laboratory measurements and one for clinical use. The laboratory systems are designed to collect the highest quality spectra possible and allow different excitation/collection wavelengths to be studied. The clinical systems are designed to collect spectra via optical fibers within a few seconds and to be mobile and hospital-compatible. These systems are capable of detecting weak near-infrared Raman bands hidden in large background signals. Calibration and background subtraction procedures are described, and system performance is evaluated.

Copper Selenocyanate Formation on the Copper Anode.

The formation of CuSeCN on a copper anode was studied in 0·1 and 0·5 mol dm-3 KSeCN solutions. A thin film of polycrystalline CuSeCN was produced with a midpoint potential between the anodic and cathodic peaks for 0·1 mol dm-3 KSeCN at - 0·253 V, close to the open-circuit potential of - 0·249 V. A precursory anodic peak or shoulder was seen at about - 0·210 V, and was ascribed to the formation of a primary (barrier) film. The anodic peak currents for the primary film were linearly dependent upon the sweep rate. Potential steps into the primary film region produced current transients, with the current decaying in proportion to the reciprocal of time. The anodic peak current and potential for the secondary polycrystalline film were proportional to the square root of the sweep rate, an effect which is consistent with a model for porous film growth controlled by the resistance across the underlying barrier film. A weak Raman band due to Se-bonded CuSeCN at 2164 cm-1 was also observed in situ.

Characterization of PAN-based carbon fibres with laser Raman spectroscopy

Laser Raman spectroscopy (LRS) has been employed to characterize the structure and morphology of a series of carbon fibres, to assess the combined effects of ultimate firing temperature (UFT) and pre-graphitization drawing during manufacture and, finally, to investigate the influence of oxidative treatment upon the integrity of the fibre surface. Ten types of PAN-based carbon fibres of varying modulus, diameter and manufacturing method, were examined. All their spectral features were recorded and analysed in terms of position, bandwidth and band intensity. Low-modulus fibres, produced at low graphitization temperatures, exhibit weak and broad Raman bands in the 1200–1700 cm−1 frequency region. With the increase of the firing temperature, the spectral features sharpen and new lines appear at higher frequencies. The observed changes in the Raman spectra are discussed in detail and related to alterations in the conditions of manufacture.

Broadband (1000 cm?1) multiplex CARS spectroscopy: Application to polarization sensitive and time-resolved measurements

A multiplex CARS spectrometer based on a cw diode-pumpedQ-switched Nd: YLF laser, a broadband dye-laser and a multichannel spectrum detection system has been constructed. Excellent mode characteristics of the laser beams and high pulse repetition rate (2 kHz) have resulted in good signal-to-noise ratio achieved with a few seconds accumulation time. A 1000 cm−1 wide spectral range is covered in a single CARS spectrum with an expanded bandwidth of the dye laser. A thin-jet sampling method is used in order to avoid the phase-matching limitation. The efficient spectral intensity normalization by the reference (CCl4) nonresonant spectrum and subsequent computer fitting have been implemented. The performance of the system is demonstrated by two different experiments. First, the polarization sensitive measurements (PS-CARS) of cyclohexane show its potential for accurate Raman depolarization ratio determination and for detection of weak (overlapped) Raman bands. Second, the transient resonance CARS measurement of the lowest excited triplet state of all-trans retinal indicate its feasibility to time-resolved CARS spectroscopy of fluorescent excited states.

High pressure Raman spectra of β-Mg 2SiO 4, γ-Mg 2SiO 4, MgSiO 3-ilmenite and MgSiO 3-perovskite

The Raman spectra of polycrystalline β-Mg 2SiO 4, MgSiO 3-ilmenite and MgSiO 3-perovskite and single crystals of γ-Mg 2SiO 4 immersed in either H 2O or a methanol-ethanol mixture were measured inside a diamond-anvil cell at room temperature up to pressures as high as 0.5 Mbar. With the exception of MgSiO 3-perovskite, all these phases were investigated at pressures beyond their respective stability fields. Except for some weak Raman bands, which were not well determined at high pressures, the Raman frequencies of all four phases were observed to vary nonlinearly with pressure. The data for γ-Mg 2SiO 4 and MgSiO 3-ilmenite at high pressures are believed to be the first reported in the literature. This study failed to find any evidence of the so-called “second” order phase transition in β-Mg 2SiO 4 at high pressures and room temperature which was reported in a previous Raman study. The data of MgSiO 3-perovskite are generally in good agreement with previously published results.

Raman spectra of tetragonal lanthanide oxychlorides obtained from polycrystalline and single‐crystal samples

AbstractThe reported Raman spectra of the tetragonal [lead chlorofluoride‐type structure, D‐P4/nmm (ITC No. 129)] lanthanide oxychlorides are inconsistent. Nuclear site symmetry analysis predicts six Raman bands. Five peaks are consistently observed, whereas the sixth remains controversial. Also, two different sets of irreducible representations are reported: which include the three acoustic modes, A2u{T z} and Eu{Tx, Ty}. The present analysis of the polarized Raman spectra from PrOCI and NdOCI single crystals clearly showed that the correct irreducible representation contains the B2g symmetry. The symmetries of three Raman‐active bands are well determined, a fourth band displays two polarizations and a fifth band has no clear polarization and is assigned Eg by default. The EuOCl Raman spectrum exhibits a weak electronic Raman band which originates in electron‐phonon coupling as a result of the closeness between the vibrational and electronic eigenvalues. The low‐energy band (LaOCl, 67 cm−1; EuOCI, 70 cm−1; and GdOCl, 65 cm−1) and the high‐energy band (525 cm−1 for LaOCl) reported by others for selected lanthanide oxychlorides were not observed in the present work.

Phase transitions and Raman spectra of anatase and rutile at high pressures and room temperature

High-pressure phase transformations of both anatase and rutile at room temperatre have been studied under an hydrostatic environment up to 115 kbar Raman spectroscopy. Anatase transforms to a columbite-type TiO 2 at 54±2 kbar, and the transition is nearly instantaneous but irreversible. Rutile transforms to the same high-pressure phase, starting near 100 kbar, but the transition is sluggish and irreversible. The pressure dependences of the various Raman bands of anatase and rutile observed in the present study are in good agreement with those published in the literature. New data of the pressure dependences of the various Raman bands of columbite-type TiO 2 are reported here. Each of these phases has one very weak Raman band and its wavenumber decreases linearly with increasing pressure. These weak bands appear to vanish (or are undetectable) at high pressures in anatase and the columbite-type TiO 2

Spectral properties and isomerism of nitroenamines. Part 3

Vibrational, NMR and dynamic NMR spectra, considered together with the results of theoretical studies, provide a complete and fairly accurate quantitative picture of the isomerism affecting the nitroenamines R2R3N–C(1)R1C(2)H–NO2(R1= H, Me). The compounds with primary or secondary amino groups (R2 and/or R3= H) exist as solvent-dependent equilibrium mixtures of the intramolecularly hydrogen-bonded Z-form and the E-form; the latter isomer can adopt the Z and/or the E conformation around the C(1)–N single bond when R2≠ R3. The compounds with a tertiary amino group exist solely in the E-form. Vibrational couplings occur inside the mesomeric system leading to an IR strong (medium or weak Raman)‘enamine’ band at 1650–1550 cm–1, the result of the asymmetrical coupling of the CC and C(1)–N stretching modes, and when R1 and R2= H, with contributions of the in-plane N–H and C(1)–H bending modes. The N–O stretchings do not contribute to the enamine band, but couple with other vibrations to give a weak IR and Raman band at 1530–1480 cm–1, with a main contribution of the νa(NO2), and a strong IR (medium or weak Raman) band, mainly, νs(NO2), at 1280–1230 cm–1. The energy barriers to rotation around the C(1)C(2) and C(1)–N bonds, and the ΔG‡ values for the ionization of the N–H group, indicated that the E⇌Z isomerization takes place by a thermal mechanism with dipolar transition state, with the contribution, in some of the compounds with an NH group, of an anionic mechanism.

Characterization of vivianite from Catavi, Llallagua Bolivia

Vivianite from Catavi Mine, Llallagua, Bolivia, has a near ideal composition with traces of Mg, Zn and Mn. Total rare-earth elements are < 1,gmg/g. Mossbauer spectroscopy shows FeIII/(FeII + FeIII) is approximately 0.04.a = 10.030A,b = 13.434A,c = 4.714A,β = 102.73dg. The middle-infrared powder spectrum shows H2O-related bands at 3490, 3290, 3130 cm−1 (stretch), 1618 cm−1 (bend), 825 cm−1 (rock), and at 665 cm−1 a possible M-OH2 twist. P04 bands occur at 1045-940 cm−1 (stretch) and 570-450 cm−1 (bend). Corresponding laser Raman microprobe bands occur at 1051 (ms), 986 (m), 948 (vs), 867 (mw), 828 (w), 568, 532, 453 (m), 442 (mw). Weak Raman bands at about 342, 303, 270 (w), 235 (ms), 227 (sh, ms), 196 (ms), 187 (sh, m), 162 (mw), and 126 (m) may arise from lattice vibrations. Differential thermal responses include a major endotherm from 115–235°C with a shoulder at 170°C and a maximum at 210°C resulting from loss of structural water combined with oxidation of Fe2+, and two small exotherms with maxima at 605 and 780°C related to structural transformations.

Raman spectroscopic study of complex formation between dimethyl sulfoxide and chloroform

AbstractThe sulfur‐oxygen stretching region of dimethyl sulfoxide (DMSO) in chloroform solutions has been studied by means of Raman spectroscopy. The number of linearly independent spectral components in the SO stretching band envelope was determined to be four by factor analysis. The overall band envelope was fitted with five Lorentz‐Gaussian functions using a non‐linear least‐squares procedure. The results were found to be consistent with the formation of 1:1 and 1:2 DMSOCHCl3 complexes, which give rise to bands centered at 1054 and 1032 cm−1, respectively. Equilibrium constants could not be calculated because of coincidences between bands of pure DMSO and bands due to the two DMSOCHCl3 complexes. The spectrum of a 0.0065 mole fraction DMSO solution in chloroform indicates a nearly complete conversion of DMSO species to the 1:1 chloroform complexes at this concentration. A very weak Raman band at 1018 cm−1 and an IR band of medium intensity at 1017 cm−1 in the spectra of pure DMSO were detected by the use of band‐fitting techniques. The assignments of these bands and the other bands in the methyl rocking region are discussed.

Vibrational study of mixed SiO 2-GeO 2 glasses

This paper presents the results of the Fourier transform infrared (FTIR) and Raman spectroscopic study of glasses in the system : SiO 2(GeO 2)-Al 2O 3-Fe 2O 3-K 2O. The prominent features in the FTIR spectra are intense, broad bands in the 800–1200 cm −1 region due to antisymmetric stretching motions of SiO 4 and GeO 4 tetrahedra. The wavenumbers of these bands varies with composition of the glass (SiO 2/GeO 2 ratio) and they correspond to very weak Raman bands. Dominant Raman bands were found in the 400–600 cm −1 region and are mainly due to a stretch-bend mixed vibration. We note that intensity of these bands appear to remain constant for glasses with different SiO 2/GeO 2 ratio. We suggest that the changes observed in the vibrational spectra of SiO 2-GeO 2 glasses may reflect the effect of structural substitution of Ge 4+ for Si 4+ in the glass network. A more precise and detailed investigation will be needed to provide a better understanding of the spectra of the analysed glasses.

Application of Waveguide Raman Spectroscopy to High-Index Dielectric Films

Waveguide Raman spectroscopy has been applied to the analysis of single-layer, refractory-oxide, thin-film coatings on fused silica. With the use of the film as a waveguide, the interaction of the laser probe beam with the film is maximized, and interference from the silica substrate is minimized. An amorphous film of Ta2O5 was found to be an excellent waveguide, yielding an intense Raman spectrum. Even though polycrystalline films of Y2O3, ZrO2, HfO2, and ThO2 were found to be poor waveguides, they still yielded Raman spectra containing useful structural information. Such Raman spectra showed that the ThO2 film was initially in an unusual structural form, which spontaneously transformed into cubic ThO2. Even for films yielding relatively weak Raman bands, substrate Raman scattering was not a serious interference. Representative spectra are presented, along with a brief discussion of the requirements for coupling optical beams into films with large refractive indices.

Electronic Raman and fluorescence spectroscopic studies of Eu3+‐doped A2SO4 · χH2O sulphates

AbstractIR, Raman (100–6000 cm−1), visible absorption and fluorescence spectra of Eu2(SO4)3 · 8H2O and that doped in A2SO4 · χH2O (with A = Li, Na, K, Rb or Cs) have been studied. The bands observed are analysed in terms of the internal and external vibration modes due to the SO and H2O groups and the Eu3+ electronic Raman transitions. Particular attention is given to the electronic Raman transitions for all the 7F0 → 7FJ′ (J′=1–6) levels in an Eu3+ (4f6) configuration. In this regard, several spectra at different temperatures (77–500 K for solids and 77–360 K for solutions) and concentrations (for solutions) were measured with different lines of an Ar+ laser. This automatically isolates and thus helps in the identification of the bands with different vibrational/electronic Raman, fluorescence or vibronic origins. The bands of H2O are also confirmed by deuteriation. The electronic Raman bands are too weak and in general are masked in the fluorescence. Addition of a few drops of KI in an aqueous solution has been found to depress the intensity for the fluorescence and as a result it facilitates the observation of the electronic Raman effect. The transition energies as observed for the various 7F0 → 7FJ′ Raman bands are consistent with those deduced by the 5D0.1 → 7FJ′ fluorescence and 7F0 → 7FJ′ IR absorptions. The transitions in the Raman and fluorescence processes seem to be inter‐related. The specimens containing the alkali metal cations or the deuteriated salts reveal very intense fluorescence, and only a few and weak Raman bands. The interstate coupling and vibronic decay of the Eu3+ ions through the several radiative and non‐radiative processes operative are generally responsible for these features.

Disulfate Ion as an Intermediate to Sulfuric Acid in Acid Rain Formation

The oxidation of the bisulfite ion by dissolved oxygen to produce sulfate ion involves the formation of a previously undetected intermediate. This intermediate has a fairly strong Raman band at 1090 wave numbers and a weak Raman band at 740 wave numbers, both of which are probably due to sulfur-oxygen stretches. The intermediate is proposed to be the disulfate ion S(2)O(7)(2-), which hydrolyzes into H(+) and either SO(4)(2-) or HSO(4)(2-) with a half-life of about 52 seconds at 25 degrees C.

Structure of Glasses and Melts in the Na&lt;sub&gt;2&lt;/sub&gt;O-SiO&lt;sub&gt;2&lt;/sub&gt;-TiO&lt;sub&gt;2&lt;/sub&gt; System

In order to elucidate the influence of TiO2 on the structure of sodium silicate glasses and melts, a MD (molecular dynamics) simulation using the ionic pair potential function, Born-Mayer-Huggins form, has been performed for 0.25Na2O⋅0.47SiO2⋅0.28TiO2 glass and the corresponding melt. The calculation pair correlation functions were in good agreement with a RDF (radial distribution function) curve obtained from the previous structural analysis by TOF (time-of-flight) pulsed neutron scattering. The computer-simulated configuration for the glass indicated that most of Ti ions were in 4-fold coordination, whilst the remainder were 6-fold coordination. The frequency spectrum of the glass obtained from MD simulation was in good agreement with the Raman spectra measured on glasses in the Na2O-SiO2-TiO2 system. As a result, the weak Raman band near 700cm-1 can be assigned to Ti ion in 6-fold coordination, and the strong Raman band near 900cm-1 reflects a Ti-O stretching band, which is characteristic of TiO4 tetrahedra corner-linked with SiO4 tetrahedra. The Raman spectra for glasses and melts in Na2O-SiO2-TiO2 system were measured by the high temperature Raman spectroscopic technique. A slight difference in the obtained Raman spectra was observed between the glasses and the corresponding melts, implying that the structure of glass should be similar to that of the corresponding melt.

Vinylogous systems. Part 5. Vibrational spectroscopy of the nitroenamine system

Vibrational frequencies have been assigned to those bonds which comprise the conjugated core (IIa) of a nitroenamine. The results are compared and contrasted with recent infrared studies of the aminoenone system (Ia). The analysis for (IIa) rests upon the postulate that vibrational coupling of the multiply bonded units is reinforced by the accompanying electronic flux, leading to an unprecedentedly low value (1 280–1 250 cm–1) for the symmetric nitro vibration of the NCCNO2 unit. No dramatic differences between the i.r. spectra of cis- and trans-isomers were apparent. The high degree of electronic polarisation that exists in nitroenamines is reflected in extremely weak Raman bands for all major functional groups.

Laser raman investigations of C xS and its related carbons

Carbon-sulfur nonstoichiometric compound, C x S, and its related carbons have been studied by Raman spectroscopy along with X-ray diffractometry. A partially carbonized polymer heat-treated at 600°C shows a weak Raman band at 1600 cm −1 and a broad strong band centering at 1300 cm −1. A heat treatment temperature of 1000°C transforms the Raman spectrum into two strong bands at 1600 and 1350 cm −1 characteristic of disordered carbons. However, the E 2 g2 Raman band of C x S samples prepared from the same polymer at 600°C is red-shifted to 1550 cm −1. In addition to the 1550 cm −1 band and a strong 1360 cm −1 band, a weak band at about 490 cm −1 is also observed. Based on the present results and those obtained earlier, a simple model of the C itxS material is proposed as a turbostratic carbon which consists of random layer hexagonal carbon ring networks with C=S and thiophenic C-S-C groups existing in the peripheral rings at the layer edges. For the C x S materials with higher sulfur contents, cross-linkings with -S n - groups exist between layers and/or crystallites.

Solid-state metal carbonyls. Part 1.—Single-crystal infrared and Raman spectra of the group VI hexacarbonyls

Single-crystal infrared reflectance spectra in polarised light (200–2500 cm–1) at liquid-nitrogen temperature are reported for M(CO)6(M = Cr, Mo, W). The reflectance accessory used is described. All the permitted factor-group components are identified and used to construct the first complete assignment for the v(CO) region of these solids. The general features of the mull spectra are accounted for. Both site- and factor-group splittings for the v3(Eg) and v6(T1u) molecular modes are deduced: these effects are very slight for v3 but considerable for v6. The weak Raman band near 1966 cm–1 in Mo(CO)6 was shown to be of v6 origin.In the v(M—C), δ(MCO) region the reflectance spectra allowed identification of factor-group components but the splitting is small except for v7(T1u) of Cr(CO)6.Many new bands are found in the far-i.r. region, the lowest being at 31.5 cm–1. All the permitted lattice modes are located. Components of the molecular modes v9(T1u), v11(T2g) and v13(T2u) are all present in the numbers allowed by site group rules. Combining i.r. and Raman data, they are located near 90, 110 and 65 cm–1, respectively. Bands below 60 cm–1 are attributed to translatory and libratory lattice modes.