Showing papers on "Raman spectroscopy published in 1981"

Normal mode determination in crystals

Abstract: The group theoretical methods by which the symmetries of normal modes in crystals may be determined are outlined, and a series of tables are presented to facilitate rapid determination of the selection rules for vibrational transitions. Emphasis is placed on the method of nuclear site group analysis in which the number of infrared and Raman active modes of each symmetry may be obtained without detailed analysis of the symmetry elements in the crystallographic unit cell or the construction of tables. By using the tables presented here for most cases identification of the crystallographic space group is sufficient information to allow determination of the vibrational mode selection rules by inspection. Several examples are included in which crystals are analyzed by each of the methods.

Light scattering study of boron nitride microcrystals

Abstract: Raman scattering and x-ray diffraction measurements are used to correlate finite-size effects on the Raman spectra of nonpolar vibrational modes in BN. The BN microcrystalline samples used exhibited domain sizes varying from 4.4 to 78.5 nm in the plane and 1.5 and 47.5 nm perpendicular to the planes. The Raman measurements indicated that the high-frequency ${E}_{2g}$ mode shifted to higher frequency and broadened as the crystallite size decreased. A formulation of the Raman cross section for scattering from nonpolar microcrystals is presented. The development includes evaluation of the susceptibility correlation function over a limited spatial extent. The results indicate that spectral changes are related to the phonon dispersion, and the wave-vector uncertainty is accounted for. The formulation is applied to describe the BN light scattering results, and good agreement was obtained to describe the observed shift in frequency.

Raman spectroscopic investigation of the structure of silicate glasses. III. Raman intensities and structural units in sodium silicate glasses

Abstract: The Raman scattering intensity of the 1100 cm−1 polarized band, which appears on the addition of Na2O to SiO2 glass, reaches a maximum at the disilicate composition. The intensity of the polarized band at 950 cm−1 increases sharply as the Na2O concentration increases above 30 mole %. These data were interpreted by normal mode calculations and by IR and Raman intensity calculations for the silicate anion structural units: SiO4 isolated tetrahedra, Si2O7 dimers, Si2O6 chain links, Si2O5 sheet units, and Si2O4 framework units. According to these simplified models, the polarized high frequency band is due to symmetric stretching of Si–O− nonbridging bonds and the frequency increases with degree of polymerization of the tetrahedra. The previous assignments of the 1100 cm−1 band to the symmetric stretch of tetrahedra containing one nonbridging oxygen and of the 950 cm−1 band to the symmetric stretch of tetrahedra containing two nonbridging oxygens were confirmed. The other main feature of the alkali silicate gl...

Enhanced Raman scattering by molecules adsorbed at the surface of colloidal spheroids

Abstract: The observation of SERS by molecules adsorbed at the surface of colloidal particles has led to analysis of classical electromagnetic field effects which contribute to enhancement of the Raman signals. Such effects alone can give rise to large enhancements which are sensitive to particle size, shape and optical constants. The theory is formulated in such a way that specific changes in Raman polarizability which occur upon adsorption can be incorporated, when these are known.

Raman investigation of ring configurations in vitreous silica

Abstract: Random network models of glass structures1 provide satisfactory qualitative descriptions of many properties of glasses (see reviews in refs 2–4). However, to obtain good quantitative agreement between theoretical analyses and experimental observations5–11 it is often necessary to assume that specific ring structures in the random networks have special importance. In the case of vitreous silica (v-SiO2), distributions of loops of SiO4 tetrahedra in the random network have been invoked5–9 to match calculated and experimental X-ray radial distribution functions (RDF). High resolution X-ray photoelectron spectra12 of v-SiO2 and quartz also provide evidence for the occurrence of four-, six- and higher-membered rings of SiO4 tetrahedra in v-SiO2. Raman spectra are also sensitive to local microstructures in vitreous solids. We have therefore examined the question of rings in the structure of v-SiO2 by comparing its Raman spectrum with spectra of crystalline silica polymorphs whose shortest loops contain four (coesite) and six (for example, α-quartz) tetrahedra. This comparison indicates that the sharp shoulder at 490 cm−1 in the spectrum of v-SiO2, previously attributed to a defect structure3 or a longitudinal optic mode14, can be assigned to four-membered ring structure. We discuss here possible basis for the stability of four-membered rings of SiO4 tetrahedra in v-SiO2 at ambient pressure. Possible reasons for the absence of the 490 cm−1 band in the vibrational density-of-states derived for a random network model by Bell and co-workers12,15 are discussed.

Anomalous Nature of Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-Chloranil

Abstract: A phase transition starting at 84 K and indicated by a marked color change of the sample has been found in the organic insulator tetrathiafulvalene-chloranil. Optical, infrared, and Raman measurements indicate that this is a reversible transition from a nominally neutral (N) solid to a nominally ionic (I) salt. Surprisingly, the N-I transition is not first order, but occurs over a broad temperature region (\ensuremath{\sim}30 K), in which there is a coexistence of N and I molecules.

Stimulated Raman scattering: Unified treatment of spontaneous initiation and spatial propagation

Abstract: A theory of stimulated Raman scattering (SRS) is presented which treats in a unified manner the buildup of SRS from spontaneous Raman scattering and the spatial propagation which leads to gain. Maxwell-Bloch equations describe the coupling between the Stokes field operator and the collective atomic operators, which are driven by a classical laser field under low-signal-gain conditions, where the atomic ground states and laser field remain undepleted. An analytic expression is derived for the forward-propagating Stokes field operator in a one-dimensional spatial approximation when the Fresnel number of the excited medium is unity. Steady-state and transient Stokes intensities are evaluated under low-gain (spontaneous) and high-gain (stimulated) conditions. The Stokes intensity is found to be exactly independent of a laser bandwidth arising solely from phase fluctuations. The power spectrum of steady-state SRS is evaluated, giving a gain-narrowed spectral profile when the laser is narrow band, and a spectrum identical to the laser spectrum when the laser is broad band. A strong analogy between SRS and two-level superfluorescence is found; in both cases the macroscopic, collective dipole moment is initiated by quantum fluctuations.

Raman and IR absorption spectroscopic studies on α, β, and amorphous Si3N4

Abstract: Vibrational excitations of α- and β-Si 3 N 4 were studied by Raman and infrared absorption spectroscopies. Group theory was employed to interpret the observed spectra of crystalline α- and β-Si 3 N 4 . Both Raman and IR spectra of amorphous Si 3 N 4 exhibit two broad peaks centered at about 400 and 900 cm −1 . Amorphous Si 3 N 4 was found to be of the non-molecular, random-network type.

Study of nickel-molybdenum-.gamma.-aluminum oxide catalysts by x-ray photoelectron and Raman spectroscopy. Comparison with cobalt-molybdenum-.gamma.-aluminum oxide catalysts

Abstract: Several series of nickel (or Co) molybdenum oxide catalysts supported on ..gamma..-Al/sub 2/O/sub 3/ have been studied by X-ray photoelectron and Raman spectroscopy. The combination of these two techniques is a complementary approach to the alumina coverage, the nature of the deposited species, and the interaction between these species. In particular, the characteristics of the monolayer coverage by the nickel species appear somewhat different from cobalt in systems without molybdenum. When molybdenum is present, formation of a phase similar to (a) NiMoO/sub 4/ is observed. No bulk NiO or Ni(OH)/sub 2/ phases are detected. Comparison with Co-Mo or Ni-Mo-..gamma..-Al/sub 2/O/sub 3/ industrial catalysts is also presented.

Coherent Raman spectroscopy

Abstract: A review of the salient features of five coherent Raman techniques is given, including typical spectra produced by each technique. The resonant and nonresonant signal contributions in the monochromatic plane wave limit are calculated for: (1) Coherent AntiStokes Raman Spectroscopy(CARS); (2) a polarization technique referred to as ASTERISK; (3) Raman-induced Kerr-effect Spectroscopy(RIKES); (4) Optically Heterodyned RIKES (OHD-RIKES); (5) Stimulated Raman Spectroscopy (SRS). The relevant noise contribution to each of these techniques is developed within the framework of a comparative signal-to-noise analysis, and realistic detection limits are discussed. The OHD-RIKES technique is selected as the most viable of the coherent Raman techniques which satisfies the following criteria: (A) suppression of nonresonant background signals and enhanced signal-to-noise ratio; (b) simplicity of operation and interpretation of results. This is the first known application of optical heterodyne detection and optimization to coherent Raman spectroscopy, and the principles developed are generally applicable to all forms of third-order nonlinear spectroscopu.

Raman scattering and structure of normal and supercooled water

Abstract: Raman spectra in the O–H stretching band in normal and supercooled water have been investigated. Spectra are taken as a function of temperature in the range +95 to −24°C. Absolute Raman cross sections corrected for instrumental and physical factors are obtained. It is shown that the isotropic O–H stretching bands in the range +20 to +95 °C are due to the contribution of ’’open’’ (or tetrahedrically bonded) and ’’closed’’ water that are temperature independent. The percentage of open water α(T) is connected and interpreted in the frame of the Stanley site percolation model. As a consequence, the isobestic point is obtained. In the supercooled region, a third spectral contribution arises, which is centered near the main peak of the isotropic O–H band of ice. Such a contribution is tentatively explained as the presence in supercooled water of true ice, as a heterophase fluctuation.

Airborne simultaneous spectroscopic detection of laser-induced water Raman backscatter and fluorescence from chlorophyll a and other naturally occurring pigments

Abstract: The airborne laser-induced spectral emission bands obtained simultaneously from water Raman backscatter and the fluorescence of chlorophyll and other naturally occurring waterborne pigments are reported here for the first time. The importance of this type data lies not only in its single-shot multispectral character but also in the application of the Raman line for correction or calibration of the spatial variation of the laser penetration depth without the need for in situ water attenuation measurements. The entire laser-induced fluorescence and Raman scatter emissions resulting from each separate 532-nm 10-nsec laser pulse are collected and spectrally dispersed in a diffraction grating spectrometer having forty photomultiplier tube detectors. Results from field experiments conducted in the North Sea and the Chesapeake Bay/Potomac River are presented. Difficulties involving the multispectral resolution of the induced emissions are addressed, and feasible solutions are suggested together with new instrument configurations and future research directions.

Raman efficiency measurements of graphite

Abstract: The Raman efficiency of graphite, germanium and silicon have been measured at room temperature with 5145 A laser excitation The efficiencies were determined from a comparison with the 1332 cm−1 zone center phonon of diamond For graphite, the 1585 cm−1 E2g intralayer mode was studied, while for Si and Ge the 525 cm−1 and 303 cm−1 F2g zone center modes were measured

Matrix isolation spectroscopy

Abstract: 1 The history of matrix isolation spectroscopy- Section A - Techniques- 2 Infrared and Raman matrix isolation spectroscopy- 3 Electronic spectroscopy of matrix isolated solutes- 4 Magnetic circular dichroism - matrix isolation spectroscopy- 5 Electron spin resonance studies of radicals trapped in rare-gas matrices- 6 Moessbauer spectroscopy on matrix-isolated species- 7 Time and frequency resolved vibrational spectroscopy of matrix isolated molecules: Population and phase relaxation processes- 8 Stable molecules- 9 Generation and trapping of unstable solutes in low temperature matrices- 10 The characterisation of high temperature molecules using matrix isolation and vibrational spectroscopy- 11 High pressure studies- 12 Non-traditional matrix isolation: adducts- Section B - Matrix Effects- 13 Interpretation of infrared and Raman spectra of trapped molecular impurities from interaction potential calculations- 14 Matrix induced changes in the electronic spectra of isolated atoms and molecules- 15 Matrix effects studied by electron spin resonance spectroscopy- 16 Molecular motion in matrices- 17 Vibrational band intensities in matrices- Section C - Applications- 18 Matrix isolation spectroscopy of metal atoms and small clusters- 19 Vibrational spectra of matrix isolated gaseous ternary oxides- 20 Matrix isolation spectra (IR, Raman) of transition metal compounds- 21 Metal carbonyls - structure, photochemistry, and IR lasers- 22 Matrix isolation vibrational spectroscopy on organic molecules- 23 Conformational isomerism studied by matrix isolation vibrational spectroscopy- 24 Hydrogen bonding in matrices- Author Index

Vibrational dynamics in 30 Si-substituted vitreous SiO 2

Abstract: We report the polarized Raman spectra of vitreous $^{30}\mathrm{Si}$${\mathrm{O}}_{2}$ and compare the principal frequencies with those of vitreous $^{28}\mathrm{Si}$${\mathrm{O}}_{2}$ This yields the $^{28}\mathrm{Si}$\ensuremath{\rightarrow}$^{30}\mathrm{Si}$ isotopic shifts for nearly all vibrational modes of the pure glassy material The shifts of the high-frequency (infrared-active) modes are as predicted by a nearest-neighbor central-force ideal continuous-random-network model originally due to Sen and Thorpe The dominant Raman line shows a modest but significant dependence on the Si mass, and this points to an effect of disorder not included in the theory The Raman-active "defect" lines exhibit essentially no Si mass dependence, which is consistent with their recent assignment to modes of highly ordered (planar) rings of bonds, and inconsistent with numerous other (broken-bond and wrong-bond) models

Raman spectra of binary high-silica glasses and fibers containing GeO2, P2O5 and B2O3

Abstract: The Raman spectra of binary high-silica glasses have been studied. The main peaks at 808 cm −1 and 710 cm −1 in vitreous B 2 O 3 and vitreous P 2 O 5 , respectively, are greatly reduced in binary high-silica glass, whereas a peak at 425 cm −1 due to GeOGe vibration and a peak at 1320 cm −1 due to P = O vibration remain strong, increasing in intensity with decreasing SiO 2 concentration. In the stimulated Raman spectra of a P 2 O 5 -SiO 2 glass fiber pumped by a mode-locked and Q-switched Nd:YAG laser at 1.064 μm, strong Stokes emissions due to the P = O vibration have been observed at 1.24 μm and 1.48 μm. In the spectra for a GeO 2 -SiO 2 glass fiber, four narrow-width Stokes emissions due to the GeOGe vibration have been observed at 1.115, 1.172, 1.235 and 1.305 μm.

Vibrational spectra and structure of fluorozirconate glasses

Abstract: A series of binary fluorozirconate glasses have been prepared in the ZrF4–BaF2 system, containing between 52–74 mol % ZrF4. Their infrared and polarized Raman spectra are reported for the first time, down to 33 cm−1. The glass transition and crystallization temperatures were also measured. The vibrational modes have been assigned to different types of atomic motions in the glass network. The basic structure of the fluorozirconate glasses is proposed to consist of zigzag chains of ZrF6 octahedra, cross linked by Ba–F ionic bands. The interochtahedral Zr–F–Zr angle distribution was estimated to peak at ∼136°, based on the analysis of the two highest frequency Raman modes.