Raman, 318, 584, 689. Raman


Vibrational spectra of adsorbed species may also be obtained by Raman spectroscopy (see Refs. 75, 76, and 78). Here, irradiation in the wavelength region of an adsorbate electronic absorption band can show features representing concomitant changes in vibrational states, or a resonance Raman effect. Figure XVI-5 shows spectra obtained with p-nitrosodimethylaniline, NDMA, as the absorbing or probe molecule, which was adsorbed on ZnO [79]. The intensity changes of the 1600, 1445, and 1398 cm peaks indicated that on adsorption of NH3, the NDMA was displaced from the acidic sites that it originally occupied.  [c.584]

The bands on the low frequency side of the excitation frequency (Vq - are referred to as the Stokes lines, consistent with the temiinology used in fluorescence, whereas those on the high frequency side (Vq + v ) are the anti-Stokes lines. It is a bit unfortunate that tliis temiinology was chosen, since the Raman process is fiindamentally different from fluorescence. In particular, fluorescence is the result of a molecule absorbing light, undergoing vibrational relaxation in the upper electronic state, and re-emitting a photon at a lower frequency. The timescale for fluorescence is typically of the order of nanoseconds. The Raman process, on the other hand, is an instantaneous scattering process that occurs on a femtosecond timescale. The photon is never absorbed by the molecule. It is usually clear whether fluorescence or Raman scattering is being observed, but there are situations where it is ambiguous. We shall not pursue the issue any fiirther here, however.  [c.1159]

The two absorption bands, at 1050 and 1400 cm , which appear in the Raman spectra of solutions of nitric acid in concentrated sulphuric acid are not attributable to either of the acid molecules. In oleum the lower band appears at 1075-1095 cm. That these bands seemed to correspond to those in the spectra of anhydrous nitric acid and solid dinitrogen pentoxide caused some confusion in the assignment of the spectrum. The situation was resolved by examining the Raman spectra of solutions of nitric acid in perchloric or selenic acids , in which the strong absorption at 1400 cm is not accompanied by absorption at about 1050 cm . Thus, the band at 1400 cm arises from the nitronium ion, and the band at about 1050 cm can be attributed in the cases of nitric acid and solid dinitrogen pentoxide to the nitrate ion formed according to the following schemes  [c.13]

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail  [c.80]

Ralulac Raman Raman effect  [c.840]

Historically, the development of the acrylates proceeded slowly they first received serious attention from Otto Rohm. AcryUc acid (propenoic acid) was first prepared by the air oxidation of acrolein in 1843 (1,2). Methyl and ethyl acrylate were prepared in 1873, but were not observed to polymerize at that time (3). In 1880 poly(methyl acrylate) was reported by G. W. A. Kahlbaum, who noted that on dry distillation up to 320°C the polymer did not depolymerize (4). Rohm observed the remarkable properties of acryUc polymers while preparing for his doctoral dissertation in 1901 however, a quarter of a century elapsed before he was able to translate his observations into commercial reaUty. He obtained a U.S. patent on the sulfur vulcanization of acrylates in 1912 (5). Based on the continuing work in Rohm s laboratory, the first limited production of acrylates began in 1927 by the Rohm and Haas Company in Darmstadt, Germany (6). Use of this class of compounds has grown from that time to a total U.S. consumption in 1989 of approximately 400,000 metric tons. Total worldwide consumption is probably twice that.  [c.162]

Uses. Rhenium finds use as filaments in electron tubes, light bulbs, and in photoflash bulbs. It is particularly useful for filaments used in mass spectrometers. The high melting point and low vapor pressure of rhenium are prerequisites to this high temperature usage. Alloys of molybdenum or tungsten are used as heating elements, but this use requires either evacuated systems or an inert atmosphere because rhenium is oxidized by atmospheric oxygen at high temperatures. About 8% of the rhenium produced is used in these appHcations.  [c.163]

Up to ca 0.6 wt % sodium dissolves readily ia mercury to form amalgams that are Hquid at room temperature (169). The solubiUty of sodium ia mercury is ca 1 wt % at 70°C (169) and 2 wt % at 140°C (37). Alloys containing >2 wt% sodium are brittie at room temperature. Sodium-rich amalgam may be made by adding mercury dropwise to a pool of molten sodium mercury-rich amalgam is prepared by adding small, clean pieces to sodium to clean mercury with agitation. In either case an iaert atmosphere must be maintained, and the heat evolved must be removed. SoHd amalgams are easily broken and powdered, but must be carefully protected against air oxidation. Amalgams are useful ia many reactions ia place of sodium because the reactions are easier to control (169).  [c.170]

Many cities of the world do not levy a separate fee on water distributed, and even in those places where water is in shortest supply, a minimal ration may be free to everyone. The problem of wasted water and unmetered water a dding to the overall water demand is not new. In ancient Rome, fountains were coimected to the pubHc water by privately installed and owned lead pipelines, many of which were unrecorded, illegal, and hence untaxed. Frontius, the water commissioner of Emperor Nerva of Rome in AD 96, developed cmde meters to increase revenue and cut demand.  [c.236]

Manufactured graphite is semimetallic ia character with the valence and conduction bands ovedappiag slightly (4—6). Conduction is by means of an approximately equal number of electrons and holes that move along the basal planes. The resistivity of single crystals as measured ia the basal plane is approximately 0.40 fifl-m-, this is several orders of magnitude lower than the resistivity across the layer planes (7—9). Thus the electrical conductivity of formed graphite is dominated by the conductivity ia the basal plane of the crystallites and is dependent on size, degree of perfection, orientation of crystallites, and on the effective carbon—carbon linkages between crystallites. Manufactured graphite is strongly diamagnetic and exhibits a Hall effect, a Seebeck coefficient, and magnetoresistance. The green carbon body is practically nonconductive however, heat treatment at 800°C decreases the resistivity by several orders of magnitude, and thereafter resistivity decreases slowly. After graphitization to over 2500°C, the room temperature electrical resistivity may range from a few hundred to a few tenths //H-m, depending on the type of raw materials used. Graphites made from petroleum coke usually have a room temperature resistivity range of 5-15 fifl-m and a negative temperature coefficient of resistance to about 500°C, above which it is positive. Graphites made from a carbon black base have a resistivity several times higher than those made from petroleum coke, and the temperature coefficient of resistance for the former remains negative to at least 1600°C.  [c.509]

Dust MIE decreases with increased temperature, hence MIE values obtained at room temperature may not apply to operations conducted at elevated temperature, such as in dryers. Test work reviewed in [59] showed that the effect of temperature is more pronounced for dusts having larger MlEs. When MIE was measured at increasing temperatures for a series of dusts having different MlEs, a double logarithmic plot of MIE (mJ) against temperature (°C) yielded straight lines which appeared to converge at 0.088 mJ and 1000°C. Dusts with larger MlEs measured at room temperature therefore had steeper slopes on the log-log graph. While there is considerable room for error when generalizing this approach, a rough guide to temperature effects could be obtained by constructing a double-log plot of MIE versus temperature and extrapolating the MIE measured at room temperature to this convergence point (0.088 mJ and 1000°C). The MIE at any intermediate temperature could then be estimated by interpolation.  [c.176]

Raman spectroscopy is primarily a structural characterization tool. The spectrum is more sensitive to the lengths, streng ths, and arrangement of bonds in a material than it is to the chemical composition. The Raman spectmm of crystals likewise responds more to details of defects and disorder than to trace impurities and related chemical imperfections.  [c.429]

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo).  [c.737]

The crosslinking systems used in silicone adhesives, coatings and sealants fall into two main categories condensation cure and addition cure systems [32,33]. Those based on condensation reaction chemistry can be divided in two sub-groups. One sub-group is the moisture condensation cure system where, as the name implies, moisture in the air is utilized for hydrolysis reactions that lead to crosslinking of the polymer chains. These crosslinking reactions proceed from the surface into the bulk and generally take place at room temperature. The second sub-group is based on direct condensation reactions between polymers with different functional groups. These latter cure systems offer adhesive with fast rate of deep section cure at both room and elevated temperature. The addition cure systems can also be divided into two sub-groups, which depend on the source of energy used to trigger the reaction i.e. heat or UV radiation. The addition cure systems have been developed for rapid processing and fast rate of deep section cure.  [c.682]

Infrared and Raman spectroscopy each probe vibrational motion, but respond to a different manifestation of it. Infrared spectroscopy is sensitive to a change in the dipole moment as a function of the vibrational motion, whereas Raman spectroscopy probes the change in polarizability as the molecule undergoes vibrations. Resonance Raman spectroscopy also couples to excited electronic states, and can yield fiirtlier infomiation regarding the identity of the vibration. Raman and IR spectroscopy are often complementary, both in the type of systems tliat can be studied, as well as the infomiation obtained.  [c.1150]

For any molecule where stmcture and hence symmetry point group are known, the appHcation of the selection rules allows the prediction of the number of fundamental Raman- and infrared-active vibrational modes, as weH as which modes can be observed in both spectra. Generally, vibrations of highly polar moieties, such as the hydroxyl group, are weak in the Raman and strong in the infrared. C bond H and C dbond C stretches and aromatic ring breathing are frequently strongly Raman active. Carbonyl stretches and C bond C stretches have moderate Raman intensity.  [c.208]

CO. + CO. = CO. with wavevector = -k. + k. + k. = k.. Their fidl degeneracy is evident since all four fields carry the same frequency (apart from sign). Resonances appear in the electric susceptibilities when, by choice of incident colours and their signs, one or more of their energy denominators (s in iiumber at. sth order) approaches a very small value because the appropriate algebraic colour combination matches material energy gaps. All Raman spectroscopies must, by definition, contain at least one low frequency resonance. When using only optical frequencies, this can only be achieved by having two fields acting conjugately and possessing a difference frequency that matches the material resonance. Further, they must act in the first two steps along the path to the third order polarization of the sample. These first two steps together prepare the Raman resonant material coherence and can be referred to as the doorway stage of the Raman 4WM event.  [c.1185]

Of the four possible WMEL diagrams for each the and doorway generators, only one encounters the Raman resonance in each case. We start with two parallel horizontal solid lines, togedier representing the energy gap of a Raman resonance. For ket evolution using, we start on the left at the lowest solid line (the ground state, g) and draw a long solid arrow pointing up (+co ), followed just to the right by a shorter solid arrow pointing down (-CO2) to reach the upper solid horizontal line, / The head of the first arrow brings die ket to a virtual state, from which the second arrow carries the ket to the upper of the two levels of the Raman transition. Since the bra is until now unchanged, it remains in g ((g ) this doorway event leaves the density matrix at second order off-diagonal in which is not zero. Thus a Raman coherence has been established. Analogously, the doorway action on the ket side must be short solid arrow down (-012) from g to a virtual ket state, then long arrow up (+C0j) to /from the virtual state. This evolution also produces. Both doorway actions contain the same Raman resonance denominator, but differ in the denominator appearing at the first step the downward action is iidierently anti-resonant ( N for nonresonant) in the first step, the upward action is potentially resonant ( R for resonant) in the first step and is therefore stronger. Accordingly, we distinguish these two doorway events by labels and respectively (see figure BL3.2. In resonance Raman spectroscopy, this first step in is fiilly resonant and overwhelms D-. (The neglect of D- is known as the rotating wave approximation.) It is unnecessary to explore the bra-side version of these doorway actions, for they would appear in the fiilly conjugate version of these doorway events. Each of the doorway steps,  [c.1188]

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section.  [c.1197]

In more realistic models of intermolecular interactions, the force on each particle will change whenever the particle changes its position, or whenever any of the other particles with which it interacts changes position. The first simulation using continuous potentials was of argon by Rahman [Rahman 1964], who also performed the first simulation of a molecular liquid (water) [Rahman and Stillinger 1971]) and made many other important methodological contributions in molecular dynamics. Under the influence of a continuous potential the motions of all the particles are coupled together, giving rise to a many-body problem that cannot be solved analytically. Under such circumstances the equations of motion are integrated using a finite difference method.  [c.369]

Table 7.1 presents us with something of a dilemma. We would obviously desire to explore i much of the phase space as possible but this may be compromised by the need for a sma time step. One possible approach is to use a multiple time step method. The underlyir rationale is that certain interactions evolve more rapidly with rime than other interaction The twin-range method (Section 6.7.1) is a crude type of multiple time step approach, i that interactions involving atoms between the lower and upper cutoff distance remai constant and change only when the neighbour list is updated. However, this approac can lead to an accumulation of numerical errors in calculated properties. A more soph sticated approach is to approximate the forces due to these atoms using a Taylor seri< expansion [Streett et al. 1978]  [c.377]

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography.  [c.135]

Figure 6.15 shows the infrared spectrum of x-tranx-crotonaldehyde, illustrated in Figure 6.16, and Figure 6.17 shows the laser Raman spectrum. The infrared spectrum is mostly of a solution in carbon tetrachloride but partly of a thin film of the pure liquid in the region where carbon tetrachloride itself absorbs. The Raman spectrum is of the pure liquid. Table 6.4 records the vibration wavenumbers of all 27 normal modes, together with an approximate description of the vibrational motions. A comparison of this table with Table 6.3 shows that vi, V2, V5, ve, V7 and V15 are all well-behaved group vibrations. A comparison of the infrared and Raman spectra shows many similarities of intensities but also some large differences. For example, V15, the C—CFI3 stretching vibration, is strong in the infrared but very weak in the Raman whereas V3, the CFI3 antisymmetric stretching vibration, is very strong in the Raman but weak in the infrared.  [c.159]

I. M. Borisov, Yu. S. Zimin, and V. S. Martem yanov, I. Vyssh. Echebn. Raved, Rhim. Rhim. Tekhnol 34(10), 46—49 (1992).  [c.348]

Co is very high, 1131°C, and provides a reasonable margin for the amount of an additional element by which usually decreases. It is also tme, at least for bulk material, that adding Cr to Co can finally make the hep stmcture become unstable and at certain compositions it is also possible that two crystal stmctures (fee and hep) are present. Furthermore it has been shown that even two different phases, one with a high Co-rich composition (ferromagnetic) and the other with a high Cr-rich content (paramagnetic) can be formed (68). Consequently, a dding Cr to Co has two important effects reduction of the and Af. For recording appHcations these values should be optimized. The must not be too close to room temperature, because then the magnetic behavior becomes too sensitive for temperature variations. Af should have a certain value because otherwise the information caimot be read by the head. The physics behind the reduction of Af and are compHcated and not completely known. However, the most usefljl model when Cr is added to Co is to consider that the magnetic moment of Co atoms is reduced by electron transfer to its 3t7band from Cr. It has been shown experimentally that the drastically decreases with the Cr content and becomes paramagnetic just above about 22 wt % at room temperature. This is not expected if Cr only acts as a simple dilutant (69). Also, the transfer of As electrons from Cr to the >d shell of Co may lower the magnetic moment (70). Furthermore, pure Cr is antiferromagnetic at room temperature and a ferromagnetic sublattice coupling also seems to be an acceptable explanation for the relative strong decrease of when compared with other X elements, which form an hep phase with Co (71). Adding Cr to Co also gives an enhanced anisotropy field. Furthermore, with the variation of the Cr content it is possible to adjust  [c.182]

Surface-Enhanced Raman Spectroscopy. A second technique for increased sensitivity uses the strong enhancement of the electric field of a light wave at certain rough metal surfaces. This surface-enhanced Raman scattering (sers) results in a 10 —10 increase in signal of molecules in contact with the surface. Submonolayers are easily observed, and solution detection limits are 10 to 10 M for ordinary Raman scatterers. The coinage metals, ie, gold, silver, and copper, are most commonly used as the rough surface because their surface electromagnetic states can be excited using visible or near-infrared lasers. The presence of sers has been observed at electrodes, coUoids (qv), and metal-island films. Several reviews detailing enhancement theories and recent appHcations of sers have been pubHshed (21).  [c.210]

Various studies have been carried out to understand the nature and function of rhenium catalysts (9). Exafs spectroscopy has been used to determine the chemical composition of various rhenium species on the surface of a MgO support. A catalyst formed by deposition of single rhenium atoms on the support surface was active for olefin hydrogenation but not cyclopropane hydrogenolysis whereas a catalyst having Re units serves both functions. These Re catalysts are made by deposition of rhenium cluster complexes on the support. The use of rhenium on y-Al202 may be favored over the use of Groups 8—10 (VIIIB) metals because rhenium has a higher affinity toward the oxygen atoms on the support surface.  [c.163]

Applications. The broad range of Raman analytical appHcations is covered elsewhere (200,202) (see Infrared technology and RAMAN SPECTROSCOPY, RAMAN SPECTROSCOPY). Its suitabitity for aqueous solutions has led to important biological appHcations (228,229). Low frequency vibrational modes important in inorganic and organometaUic chemistry can often be studied more easily by Raman than by far-ir spectroscopy. Using modern instmmentation Raman spectra can be recorded to within a few cm of the exciting line. Sampling ease and the abiHty to probe through packaging materials makes ft-Raman useful in industries such as pharmaceuticals (230).  [c.319]

Production of carbon disulfide expanded rapidly after World War II to supply the growing needs of the viscose rayon industry, which consumes about 0.31 ton CS2 per ton rayon. The high plant capacities obtainable with the methane—sulfur route resulted in consoHdation of the carbon disulfide industries in the United States and Western Europe, where a few producers now account for the bulk of the capacity (see Table 3). Some rayon manufacturers produce their own carbon disulfide. Rayon enjoys an extensive international market that can affect local CS2 manufacturers. Competition from nonceUulosic synthetic fibers has caused a drop in rayon production in the United States since the mid-1960s. One rayon plant in the United States closed in 1989 as a result of environmental concerns. However, plans have been announced to build a new rayon plant in Louisiana (130) that is expected to achieve improved carbon disulfide utilization and low emissions by recovering and recycling carbon disulfide. This pattern of modem viscose rayon plants replacing aging faciUties that cannot be economically upgraded is apt to be repeated in other parts of the world. In a development that could have far-ranging implications, a viscose rayon producer is constmcting a solvent spun ceUulosic fiber plant using an amine oxide solvent rather than carbon disulfide (131,132).  [c.32]

To a good approximation, thermal conductivity at room temperature is linearly related to electrical conductivity through the Wiedemann-Eran2 rule. This relationship is dependent on temperature, however, because the temperature variations of the thermal and the electrical conductivities are not the same. At temperatures above room temperature, thermal conductivity of pure copper decreases more slowly than does electrical conductivity. Eor many copper alloys the thermal conductivity increases, whereas electrical conductivity decreases with temperature above ambient. The relationship at room temperature between thermal and electrical conductivity for moderate to high conductivity alloys is illustrated in Eigure 5.  [c.222]

Toluene is a notoriously poor electrical conductor even in grounded equipment it has caused several fires and explosions from static electricity. Near normal room temperature it has a concentration that is one of the easiest to ignite and, as previously discussed, that generates maximum explosion effects when ignited (Bodurtha, 1980, p. 39). Methyl alcohol has similar characteristics, but it is less prone to ignition by static electricity because it is a good conductor. Acetone is also a good conductor, but it has an equihbrium vapor pressure near normal room temperature, well above UFL. Thus, acetone is not flammable in these circumstances.  [c.2317]

Since it is not practical to manufacture a llameproof enclosure due to its size and bulk and the number of knockouts and openings on the doors for switches, metering, indicators, and pushbuttons (PBs) etc., it is common practice to locate the.se assemblies some distance from the affected area in a separate well-ventilated room. Depending upon the location and intensity ol contamination, it may be permissible to meet the requirement by using a pressurized enclosure by maintaining a positive pressure inside the enclosure similar to that for motors (Section 7.1.3..3). When there arc many switchgear assemblies, the room itself can be pressurized, which is safer and easier. Small enclosures, however, such as a PB station, switch or a switch fuse unit or an individual starter unit etc., which can be easily made of MS plates or cast iron, as discussed in Section 7.13, can be mounted in the hazardous area while the main MCC can be installed in the control room, away from the contaminated area and from where the process can be monitored.  [c.363]

At the same time, I would not bet 3 on no rain in return for 4 if it does not rain. This behavior would be inconsistent, since if I did both simultaneously I would bet 6 for a certain return of only 4. Consistent betting would lead me to bet 1 on no rain in remrn for 4. It can be shown that for consistent betting behavior, only certain rules of probability are allowed, as follows.  [c.315]

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298].  [c.262]


See pages that mention the term Raman, 318, 584, 689. Raman : [c.299]    [c.1164]    [c.1185]    [c.1206]    [c.1214]    [c.2827]    [c.475]    [c.134]    [c.123]    [c.388]    [c.642]    [c.161]    [c.164]    [c.190]    [c.51]    [c.431]    [c.215]   
Physical chemistry of surfaces (0) -- [ c.0 ]