Raman Line

As stated in Secs. 1.8 and 1.9, it is possible, by using group theory, to classify the normal vibration into various symmetry species. Experimentally, measurements of the 

The intensity of the scattered light is proportional to the sum of squares of the individual dijEj terms. Thus, the ratio of the intensities in the y and z directions is 

In a homogeneous liquid or gas, the molecules are randomly oriented, and we must consider the polarizability components averaged over all molecular 

It is possible to obtain approximate depolarization ratios of fine powders where the molecules or ions take pseudorandom orientations (see Ref. 97). 

These two quantities are invariant to any coordinate transformation. It can be shown [3] that the average values of the squares of oty are  

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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. 

For homodyne detection, the TR-CRS intensity (for Lorentzian Raman lines) is of the fomi [115]... 

Of great interest to physical chemists and chemical physicists are the broadening mechanisms of Raman lines in the condensed phase. Characterization of tliese mechanisms provides infomiation about the microscopic dynamical behaviour of material. The line broadening is due to the interaction between the Raman active chromophore and its environment. 

Valence Vibrations. pCH and pCD. In the 3100 cm region the infrared spectrum of thiazole shows only two absorptions at 3126 and 3092 cm F with the same frequencies as the corresponding Raman lines (201-4) (Fig. I-IO and Table 1-23). In the vapor-phase spectrum of... 

The second ring vibration gives rise to a very weak infrared absorption band at 467 cm and to a weak and depolarized Raman line at 470 cm (202, 203) (Table 1-23). 

Suites 1 to VIII contain infrared frequencies corresponding to vibration-rotation bands of A, B, or (A-l-B) hybrid types and can thus be assigned to vibrations of A symmetry the corresponding Raman lines are generally polarized. 

The frequencies classified in suites IX and X belong to depolarized Raman lines and correspond to vibrations-rotation bands of the C type. They can be assigned to oscillations of A" symmetry. 

Raman Spectroscopy. Raman spectroscopy is an excellent method for the analysis of deuterium containing mixtures, particularly for any of the diatomic H—D—T molecules. For these, it is possible to predict absolute light scattering intensities for the rotational Raman lines. Hence, absolute analyses are possible, at least in principle. The scattering intensities for the diatomic hydrogen isotope species is comparable to that of dinitrogen, N2, and thus easily observed. 

Usually, particle size has relatively little effect on Raman line shapes unless the particles are extremely small, less than 100 nm. For this reason, high-quality Raman spectra can be obtained from powders and from polycrystalline bulk specimens like ceramics and rocks by simply reflecting the laser beam from the specimen surface. Solid samples can be measured in the 90° scattering geometry by mounting a slab of the solid sample, or a pressed pellet of a powder sample so that the beam reflects from the surface but not into the entrance slit (Figure 3). 

With a special optical system at the sample chamber, combined with an imagir system at the detector end, it is possible to construct two-dimensional images of the sample displayed in the emission of a selected Raman line. By imaging from their characteristic Raman lines, it is possible to map individual phases in the multiphase sample however, Raman images, unlike SEM and electron microprobe images, have not proved sufficiently useful to justify the substantial cost of imaging optical systems. 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

The Raman spectrum in Fig. 10 for solid Ceo shows 10 strong Raman lines, the number of Raman-allowed modes expected for the intramolecular modes of the free molecule [6, 94, 92, 93, 95, 96, 97]. As first calculated by Stanton and Newton [98], the normal modes in molecular Ceo above about 1000 cm involve carbon atom displacements that are predominantly tangential... 

In the low frequency region, the calculations predict nanotube-specifiic Eig and E g modes around 116 cm and 377 cm respectively, for (10,10) armchair naiiotubes, but their intensities are expected to be lower than that for the A g mode. However, these Eig and E2g modes are important, since they also show a diameter dependence of their mode frequencies. In the very low frequency region below 30 cm a strong low frequency Raman-active E2g mode is expected. However, it is difficult to observe Raman lines in the very low frequency region, where the background Rayleigh scattered is very strong. 

Hiura et al. [23] observed two Raman lines in their spectrum of nested carbon nanotubes at 1574 (FWHM = 23 cm ) and at 2687 cm. It is interesting to note that their first-order peak at 1574 cm lies between, and is more than twice as broad, as either of the two first-order lines in identi-... 

The number and intensity of the Raman lines and the magnitude of the Raman shift can be related to the identity, structure, and bonding of the molecules of the compound scattering the light. 

Laser Raman spectroscopy as it is applied to the study of surface adsorbed.species involves a number of experimental problems such as fluorescence, weak Raman lines, and interfering plasma lines. Techniques of overcoming these problems have been continually improved and good... 

The Raman spectrum of an oxide sample after adsorption may be considered to consist of the spectrum of the adsorbed species superimposed on the spectrum due to the oxide adsorbent. In general, the Raman spectra of oxide adsorbents are sufficiently weak or sufficiently simple that they allow the detection of Raman lines due to the adsorbed species. This is one major advantage of Raman scattering over infrared absorption spectroscopy. The infrared spectra of most oxide adsorbents show strong absorptions which may obscure those arising from the adsorbates (Figs. 13,14). 

For the detection of weak Raman lines, high laser power, high signal amplification, long pen period, and very slow scanning speed should be... 

Characteristic Raman Lines and Their Relative Intensities for Various Pyridine (Py)... 

Spectral changes on adsorption are of three types appearance of inactive fundamentals (often coincident with infrared absorptions—see Table IX), shifts in Raman line positions for active vibrations, changes in relative peak intensities, and changes in half-bandwidths. The first three types of change have been reported for centrosymmetric adsorbates. 

Loader 38) studied the Raman spectra of styrene adsorbed on different silicas—chromatographic grade silica gel, Cab-O-Sil, and Aerosil 380. The author utilized the fact that chemisorption will bring about marked changes in the spectrum whereas physical adsorption will cause only a broadening of the Raman lines accompanied in some cases by a frequency... 

Raman effect (continued) spectral activity, 339-341 terminology of, 295 vibrational wavefunctione, 339-341 Raman lines, 296 weak, 327-330 Raman scattering, 296 classical theory, 297-299 quantum mechanical theory, 296, 297 Raman shift, 296... 

Fig. 6. Approximate form of vibration assigned to 1616 cm"1 Raman line. Also shown are the ring axes Ox Ox2Ox3. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

The infra-red measurements were of two types, normal-film measurements with the sample sandwiched between KBr plates, and tilted-film experiments with the sample sandwiched between 45° prisms of KBr, in each case with layers of Nujol to provide optical matching. Whereas the 1616 cm 1 Raman line occurs in a region well clear of other lines so that it was satisfactory to measure peak intensities, the infra-red spectrum of PET shows many overlapping bands. Accurate assessment of absorption intensities therefore requires the computer separation of the spectrum into a set of overlapping peaks (shown to be Lorentzian in profile) and a linear background. The procedures adopted and the band assignments are discussed in detail by Hutchinson et al. 6). 

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