Showing papers on "Raman spectroscopy published in 1984"

Structural studies of silicate glasses and melts—applications and limitations of Raman spectroscopy

Abstract: Raman spectroscopy studies of alkali and alkaline earth silicate glasses and melts are reviewed, and the major Raman bands observed are summarized. To realize the full potential of Raman spectroscopy will require a detailed understanding of their vibrational properties in relation to structure. A number of vibrational calculations have been carried out to address this problem. These are briefly summarized, and some limitations of the method are noted. Systematic variations in bulk properties are examined for viscosity and the immiscibility behavior.

Size effects in the excited electronic states of small colloidal CdS crystallites

Abstract: This paper reports experimental studies of the development of bulk optical properties as a function of crystallite size for the inorganic direct gap semiconductor CdS. Small crystallites are synthesized via colloidal chemical techniques, and their optical properties are studied in situ at extreme dilution. The crystallites are characterized via high resolution transmission electron microscopy. Direct images show (111) lattice planes, and establish the crystallite structures as close to those of excised fragments of bulk CdS (zinc‐blende cubic). Large crystallites (> 100 A average diameter) show an optical absorption, in colloidal solution, close to that of bulk crystalline material. However, small crystallites of 30 A average diameter show a large blue shift (∼0.8 eV) in absorption edge (effective band gap), and an intensification of edge absorption relative to absorption at higher energy regions. These observations can be understood as quantum size effects resulting from confinement of an electron and hole in a small volume. 40 A average size crystallites show a smaller shift (∼0.25 eV), and corresponding changes in their fluorescence, and resonance Raman excitation, spectra.

Modeling studies of amorphous carbon

Abstract: Three structural models of amorphous carbon with differing percentages of threefold- and fourfold-coordinated atoms were constructed and analyzed, along with a purely four-coordinated amorphous Ge model which was scaled to diamond bond lengths. The radial distribution function and interference function, $F(k)$, of the model containing 14% four-coordinated atoms were in best agreement with the experimental results of Boiko et al., although the functions containing 48% four-coordinated atoms were in best agreement with the results of Kakinoki et al. The unavoidable planar nature of the entirely three-coordinated model caused its $F(k)$ to be in poor agreement with experiment. Raman and vibrational density-of-states (DOS) spectra were also calculated for the models. The presence of disorder in the three-coordinated model produced a downward shift in frequency of the principal Raman peak and DOS band edge from the position seen in graphite. With the addition of four-coordinated atoms, there was a gradual transition from graphitelike spectra to diamondlike spectra, rather than spectra with a mixture of distinct features typical of graphite and diamond. In addition, there was a further downward shift in frequency of the main Raman peak and in that of the "disorder peak" seen in microcrystalline and amorphous carbon. The spectra of the model containing 14% four-coordinated carbon was in best agreement with recent Raman scattering experiments. These results suggest a structure for amorphous carbon consisting of three-coordinated planar regions with occasional four-coordinated atoms allowing changes in orientation of the planes. The positions of the peaks in the spectra suggest that the proportion of four-coordinated atoms is not likely to exceed 10%.

Effects of As+ ion implantation on the Raman spectra of GaAs: ‘‘Spatial correlation’’ interpretation

Abstract: We have studied Raman scattering from 〈100〉 GaAs samples implanted with 270‐keV As+ ions with various fluences up to 3.2×1014 cm−2. In addition to phonon density of states effects, we observe a softening and asymmetric broadening of the allowed LO phonon while the small symmetry forbidden TO phonon remains almost unchanged. The behavior of the LO and TO modes can be explained quantitatively on the basis of a ‘‘spatial correlation’’ model related to q‐vector relaxation induced by the damage. Our interpretation is quite general and makes it possible to use Raman spectra to evaluate an average size of undamaged regions in semiconductors.

Surface-enhanced Raman spectrometry for trace organic analysis

Abstract: The analytical potential of a new spectrochemical method based on surface-enhanced Raman spectrometry (SERS) on silver-spheres-coated substrates is presented for the first time in this work. A new method for preparing SERS-active substrate using submicron size silver-coated spheres deposited on filter paper substrates is described in detail. The analytical figures of merit and limitations of the technique are discussed. The detection limits for several organic compounds such as carbazole, 1-aminopyrene, and benzoic acid are 0.2, 1.4, and 0.3 ng, respectively. The results indicate that SERS shows great promise as a useful analytical tool for trace organic analysis.

Development of the stimulated Raman spectrum in single-mode silica fibers

Abstract: The spectral development of stimulated Raman scattering in single-mode silica fibers was studied both experimentally and by computer modeling. The most striking feature that emerges is the rapid growth of a weak feature at 490 cm−1 at the expense of a broad band at 440 cm−1 as pump power increases. These experimental results are in good agreement with our numerical simulations, although neither experiments nor calculations show the spectral broadening of higher Stokes orders commonly observed with high pump powers and at infrared wavelengths. It is shown that, in general, spectral broadening from four-wave mixing should be important in the development of the stimulated Raman spectrum. However, the present experiments fall into a regime of relatively low pump powers at visible wavelengths in which four-wave mixing is negligible and the stimulated spectrum depends only on the shape of the Raman gain curve.

Dimagnesium iron(II) hydride, Mg2FeH6, containing octahedral FeH64- anions

Abstract: Etude par diffraction RX et de neutrons mettant en evidence une symetrie cubique et une structure type K 2 PtCl 6 . Spectres IR, Raman et Mossbauer montrant la presence d'ions [FeH 6 ] 4− octaedriques a bas spin

Characterization of DNA structures by laser Raman spectroscopy.

Abstract: Oriented fibers drawn from aqueous gels of calf-thymus DNA were maintained at constant relative humidites of 75 and 92% to yield canonical A-DNA and B-DNA structures, respectively. Raman spectra of the two forms of DNA were recorded over the spectral range 300–4000 cm−1. The authenticated DNA fibers were deuterated in hygrostatic cells containing D2O at appropriate relative humidities, and the corresponding spectra of deuterated DNAs were also obtained. The spectra reveal all of the Raman scattering frequencies and intensities characteristic of A- and B-DNA structures in both nondeuterated and deuterated froms, as well as the frequencies and intensities of adsorbed solvent molecules from which the hydration content of DNA fibers can be calculated. Numerous conformation-sensitive vibrational modes of DNA bases and phosphate groups have been identified throughout the 300–1700-cm−1 interval. Evidence has also been obtained for conformation sensitivity of deoxyribosyl CH stretching modes in the 2800–3000-cm−1 region. Raman lines of both the backbone and the bases are proposed as convenient indicators of A- and B-DNA structures. The results are extended to Z-DNA models investigated previously. Some implications of these findings for the determination of DNA or RNA structure from Raman spectra of nucleoproteins and viruses are considered.

Growth and Physical Properties of LPCVD Polycrystalline Silicon Films

Abstract: Undoped LPCVD silicon films have been deposited at five temperatures between 560° and 620°C. The films were characterized as grown and after thermal annealing at 900°, 950°, and 1000°C. We used x‐ray diffraction, TEM, SEM, Raman and elastic light scattering, optical absorption and reflection, and other techniques in order to obtain information on the grain size, structure, structural perfection, and surface roughness. We found that polysilicon films of good structural perfection, low strain, and small surface roughness are obtained when the films are deposited in the amorphous phase and subsequently crystallized at 900°–1000°C. Such films are superior in all investigated material aspects to films grown in the crystalline phase.

Raman scattering from GexSi1−x/Si strained‐layer superlattices

Abstract: Raman spectroscopy has been used to determine built‐up deformation in GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy. By comparing peak positions in commensurate superlattices and single layers with those from incommensurate thick layers of the same composition we can obtain a quantitative determination of strain. Linewidths are affected by the presence of inhomogeneous strain, dislocations, and disorder. Lines are always narrower in superlattice samples, indicating better crystalline quality. In particular, the Raman line from the Si layers of the strained‐layer superlattices is indistinguishable from that from single‐crystalline Si in both linewidth and frequency. This is consistent with the expectation that the entire lattice mismatch is accommodated as a homogeneous tetragonal strain in the alloy layers only.

A Raman spectroscopic study of glasses in the system CaO-MgO-SiO 2

Abstract: The Raman spectra of a number of glasses in the system CaO-MgO-SiOz have been prepared using solar furnace techniques. The spectra of glasses along the SiOz-CaO, SiOzMgO and SiOz-Cao.5Mgo.50 joins are similar to those obtained in other studies, and are interpreted in terms of vibrations of tetrahedral silicate units with zero, one, two, three and four non-bridging oxygens, consistent with previous discussions. Some glasses were prepared with bulk compositions within the CaO-MgO-SiOz two-liquid field, using both a "normal" and a fast quench rate. The normal-quenched glasses were opaque and unmixed, the fast-quenched samples transparent and homogeneous. Both sets of samples had identical spectra, and the implications of this are discussed. Finally, a number of samples with varying CalMg ratio at constant high silica content were studied. Their Raman spectra are discussed along with results of previous studies on alkali and alkaline earth silicate glass systems to develop a molecular site model which describes the effect of changing metal cation on the distribution of silicate species.

Raman spectroscopy of a coal liquid shows that fluorescence interference is minimized with ultraviolet excitation

Abstract: The first ultraviolet resonance Raman measurements of a coal liquid are reported. The spectra detail the presence of numerous polycyclic aromatic hydrocarbons with ring systems similar to those of naphthalene, fluorene, phenanthrene, pyrene, and triphenylene . The ultraviolet resonance Raman measurements of this highly complex sample show no significant interference from fluorescence. The lack of fluorescence interference and the high selectivity indicate that ultraviolet resonance Raman spectroscopy is a powerful new technique for characterizing highly complex samples and mixtures.

Development of a New Optical Wavelength Rejection Filter: Demonstration of its Utility in Raman Spectroscopy

Abstract: A new tunable optical filter has been developed which rejects a narrow wavelength interval ( 90%). This filter will be useful in optics, in spectroscopy, and for laser applications. The active element of the filter is a crystalline colloidal array of polystyrene spheres. The rejected wavelengths are Bragg diffracted from this ordered array. For a particular sphere concentration and scattering from a particular set of lattice planes, tunability can be achieved by the altering of the angle between the filter and incident light beam. Bragg diffraction and light rejection of these filters are monitored by transmission measurements. The utility of this filter for spectroscopic measurements is demonstrated for Raman spectroscopy. Raman measurements are shown for polypropylene, a highly scattering material with numerous low-frequency modes. The filter selectively attenuates the elastically scattered light and allows the low frequency peaks to be observed. Use of this wavelength rejection filter to reject the Rayleigh scattered light simplifies the instrumentation, decreases the cost, and increases the sensitivity of Raman spectral measurements. This filter also has the potential to replace dispersive elements such as gratings and prisms in a variety of spectroscopic and optical applications.

Versatile, efficient Raman sampling with fiber optics

Abstract: Description d'un systeme dans lequel la lumiere laser et la diffusion Raman ont pour support des fibres optiques. Cette technique permet des analyses dans des environnements hostiles

The electromagnetic theory of surface enhanced spectroscopy

Abstract: Surface enhanced spectroscopy (SES) was born in 1 974 with the measure­ ment of Raman spectra of molecules adsorbed on a roughened electrode surface (la-e). Given the smallness of the Raman cross section, the detection of a Raman signal should have generated some excitement. This did not happen, but was delayed until 1977 when Jeanmaire & van Duyne (2) and Albrecht & Creighton (3) showed that the rough silver surface enhances the Raman cross section by a factor ranging between 104 and 106. This started a great outpouring (4a-i) of theoretical and experimental work, whose central theme is to understand how the presence of a solid surface modifies the spectroscopic and photochemical properties of a molecule located nearby. We divide the effects of the surface in two categories: electromagnetic, which can be described by solving Maxwell's equations, and chemical, which belong to quantum chemistry. The enhancement of the local laser field due to the polarization of the surface is an example of an elec­ tromagnetic effect. The appearance of a new excited state caused by chemisorption, leading to an enhancement of the Raman cross section through resonance Raman scattering (which would not be expected on the basis of the gas phase properties of the molecule), is an example of a chemical effect. While there is no doubt that chemisorption modifies the optical response of adsorbed molecules, the magnitude of the modification is still a subject of controversy. We don't know whether we should expect large modifications for all molecules, or for a small class (e.g. those with 11: orbitals); or whether

Optical power limits in multi-channel wavelength-division-multiplexed systems due to stimulated Raman scattering

Abstract: General expressions are derived to estimate transmitter power limitations due to stimulated Raman scattering in wavelength-division-multiplexed optical communication systems. These results are applicable to systems containing an arbitrary number of channels with arbitrary (but equal) channel separation.

Molecular and electronic structure in the metal‐to‐ligand charge‐transfer excited states of d6 transition‐metal complexes in solution

Abstract: The ground and metal-to-ligand (MLCT) excited electronic states of the complexes Os(bpy)/sub n/(P/sub 2/)/sub 3-n//sup 2 +/ (bpy = 2,2'-bipyridine, P/sub 2/ = cis-(C/sub 6/H/sub 5/)/sub 2/PCH=CHP(C/sub 6/H/sub 5/)/sub 2/; n = 1-3) have been studied by resonance Raman spectroscopy, time-resolved resonance Raman spectroscopy, and emission spectroscopy. The time-resolved resonance Raman evidence confirms that the charge-transfer electron density is localized in the lowest ..pi..* orbital of one bpy ligand rather than delocalized over the ..pi..* orbitals of all of the available bpy ligands on the vibrational time scale. The amount of charge transferred from the metal to bpy ..pi..* in the MLCT state has been determined. Application of Badger's rule to the Raman data and Franck-Condon analysis of the emission data lead to two independent determinations of average bond length displacements in the MLCT state. The two approaches yield displacement values that agree within 0.001-0.003 A for the three complexes studied, suggesting that it may be possible to determine excited-state structures in solution with a precision silimar to that of a good X-ray crystal structure. Analysis of time-resolved resonance Raman and emission data for Ru(pby)/sub 3//sup 2 +/, Ru(bpy)/sub 2/(en)/sup 2 +/, and fac-Re(bpy)(CO)/sub 3/Cl by these methods yields resultsmore » in agreement with the osmium data. The results suggest a general prescription for the determination of the molecular and electronic structures of electronically excited states in solution. 40 references, 6 figures, 4 tables.« less

A Raman study of pressure‐densified vitreous silica

Abstract: We have obtained polarized Raman spectra for vitreous silica densified at 3.95 GPa and 530 °C. The glass spectra show considerable changes on densification, which are discussed in terms of the observed reduction in average intertetrahedral bond angle and the central force model for connected networks. The results of this densification experiment are compared with previous work at high pressure but room temperature. This comparison suggests a model for the densification mechanism, and indirectly supports recent structural interpretations of the Raman spectrum of fused silica.

Theory of Raman scattering in superconductors

Abstract: The electronic Raman scattering by pairs of quasiparticles is calculated at zero temperature, generalizing previous calculations that were based on the Bardeen-Cooper-Schrieffer model of a superconductor. Analytical and numerical results are presented for the spectrum as a function of wave vector q, and an integration is performed over q/sub z/ to include the effect of a finite optical penetration depth. Allowing for gap anisotropy, we correct the results for vertex and Coulomb polarization effects. The theoretical results for finite q are used to calculate spectra for Nb/sub 3/Sn, V/sub 3/Si, and Nb, neglecting gap anisotropy. Experimental data are presented for V/sub 3/Si and Nb. The data for V/sub 3/Si are fit to a zero-q theory that includes gap anisotropy, with results similar to those presented earlier for Nb/sub 3/Sn. The role of possible excitons on the Raman spectra is examined. These theoretical results are then used to discuss the self-energy of a Raman-active optical phonon in a superconductor.

Surface-Enhanced Raman Scattering and Nonlinear Optics Applied to Electrochemistry,

Abstract: The most recently developed diagnostic technique in metal-electrolyte and metal-gas interfaces adapts spontaneous Raman scattering and nonlinear optical generation, techniques normally applied to bulk media, to surface science investigation. For certain metallic surfaces, an enormous increase exists in the Raman (as much as 106 to 108 times) and nonlinear optical signals resulting from submonolayer coverage of molecular adsorbates at the interface. Spontaneous Raman scattering and nonlinear optical scattering are well developed in both theory and practice for the analysis of molecular structure and concentration in bulk media. Instrumentation to generate and detect these inelastically scattered signals is readily available and is adequate for adaption to surface science. However, the mechanism (or mechanisms) giving rise to such a large enhancement at the interfaces is still being actively researched and remains controversial. Theoretical and experimental investigations related to the underlying ...

The densification of monolithic gels

Abstract: The densification of gels to glasses with composition SiO 2 and 8Al 2 O 3 92SiO 2 has been studied as a function of heat treatment between 120 and 1200°C using Raman spectroscopy together with measurements of density and specific surface area. Monolithic dry gels are prepared at 120°C. In the case of SiO 2 these have a density of 1.39 g/cm 3 , a specific surface area of 790 m 2 /g and a mean pore radius of 10 A. In the case of 8Al 2 O 3 92SiO 2 gels these values are 0.76 g/cm 3 , 535 m 2 /g and 50 A, respectively. The Raman spectra of monolithic dry SiO 2 gels, measured as a function of temperature, show that both surface and bulk SiOH groups are present. The relative concentration of these groups is temperature dependent. The total OH content decreases with increasing temperature. The combined results of Raman, density and specific surface area measurements show that during densification two, partly related, processes occur, viz. pore collapse and the condensation of surface SiOH groups to SiOSi bonds.

Brillouin and raman scattering study of borate glasses

Abstract: We report light scattering results obtained from Brillouin and Raman scattering experiments in a series of B 2 O 3 − x M 2 O glasses (M  Li, Na, K, Rb, Cs and Tl). Raman lines related to the internal vibrations of structural units give information on the composition and concentration dependence of the short range arrangement in the glasses. The “Boson peak” is analysed on the basis of the Martin-Brenig model. The short correlation range is lower than 10 A and increases with the cation radius. From these experiments the very low frequency light scattering (Light Scattering Excess) is extracted. The temperature dependence of the LSE intensity in pure B 2 O 3 glass as well as its variation with composition at room temperature are studied. The Theodorakopoulos-Jackle theory relates this intensity to the acoustic attenuation deduced here from Brillouin linewidth measurements. While a theoretical description of the relation between the temperature variation of both quantities agrees well with experimental results, disagreement is apparent in their composition dependence. Finally, the sound velocity plotted versus concentration gives evidence of the contradictory influence of the hardening due to the change of the boron atom coordination number and the softening related to the distortion of the network by large cations.

Raman-Microsampling Technique Applying Optical Levitation by Radiation Pressure

Abstract: Recently, there has been a rapid development in Raman microprobe spectroscopy [1]. It was shown that Raman spectra can be obtained from micrometer-size particles with reasonable signal-to-noise ratio. In these techniques specific precaution has to be taken into account to minimize spectral contributions arising from elastic and inelastic scattering from all possible sources other than the particle of interest. Particularly, inelastic scattering from the substrate which supports the particle for measurement must be avoided in order to keep spectral interference with the Raman spectrum of interest to a minimum.