Depolarized Raman lines

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). 

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. 

The assignment of bands given in Fig. 1 are confirmed by the laser Raman spectra [4]. The vibrations assigned to Ai species appear as the polarized Raman lines and those assigned to Bi and B2 species appear as the depolarized Raman lines. The results are just expected from the above assignments except some weak lines, for which the depolarisation ratio cannot be measured definitely. 

Polarized Raman spectra of a, -unsaturated esters with a plane of symmetry help in distinguishing out-of-plane vibrations (weak depolarized Raman lines) from in-plane vibrations (medium-strong polarized Raman lines) (see Table 9.5). 

It is important from a practical viewpoint to predict the shear viscosity of mixtures from those of pure melts. For alkali nitrate melts, a linear dependence has been found between the reorientational line width obtained by Raman measurements and the ratio of temperature divided by shear viscosity.For NO3 ions, the depolarized Raman scattering from 1050cm" total stretching vibrational mode (Al) has a contribution to the line width L, which is caused by the reorientational relaxation time of the Csv axis of this ion. The Stokes-Einstein-Debye(SED) relation establishes a relation between the shear viscosity r of a melt and the relaxation time for the reorientation of a particle immersed in it ... 

A review article has been published on depolarized Raman spectroscopy in molten salts, and it is discussed briefly here for the case of molten nitrates. Polarized (VV) and depolarized (HV) Raman scattering of the 1050 cm" line was used. The intensity of the isotropic part of the... 

IR line shapes for this system have been measured at room and other temperatures. Isotropic, depolarized, and unpolarized Raman line shapes have also been measured. For this system all Raman line shapes are similar, peaking at about 3430 cm 1, with a shoulder at about 3625 cm 1 (at room temperature). The IR line shape is red-shifted by about 30 cm 1 and does not show the blue shoulder. Experimental IR [10] and unpolarized Raman [12] line shapes are shown in Fig. 3. 

In addition to the intensity and frequency of the Raman lines, the polarization character of the lines can be measured. In fact, what led Sir C. V. Raman to believe that he was observing a new phenomenon was the unique polarization properties of this new radiation. Usually, the observations are made perpendicular to the incident beam, which is plane-polarized, as at (a) in Fig. 15. The depolarization ratio p is defined as the intensity ratio of the two polarized components of the scattered light that are respectively parallel and perpendicular to the direction of the (polarized) incident beam when the polarization of the incident beam is perpendicular to the plane of propagation and observation (p = I /ll). 

Theoretically, 0=s p =s 3/4, depending on the nature and symmetry of the vibration. Nonsymmetric vibrations give depolarizations of 3/4. Symmetric vibrations give p ranging from 0 to 3/4. Accurate values of p are important for determining the assignment of a Raman line to a symmetric or an asymmetric vibration. 

Although polarization data are normally obtained for liquids and single crystals, it is possible to measure depolarization ratios of Raman lines from solids by suspending them in a material with similar index of refraction (27). 

Here, T is the observed line width (Av << F), 7d is the peak-to-valley intensity in the difference spectrum, and To is the peak height of the Raman line. Although this equation is for Lorentzian-shaped bands, the results are approximately the same for Gaussian-shaped bands (the constant 0.385 becomes 0.350). In the case of carbon disulfide-benzene mixtures, the smallest shift observed was -0.06 cm-1, and the associated error was 0.02 cm-1 (77). A convenient rotating system that can be used for (1) difference spectroscopy, (2) normal rotating sample techniques (solid and solution), and (3) automatic scanning of the depolarization ratios as a function of the wave number has been designed (45). 

The depolarization ratio of the Raman lines in the spectrum of a liquid compound, which is easy to measure, provides a means of distinguishing totally symmetrical vibrations from the rest. 

Vibrations of totally symmetric species (defined by the first row of the character tables) emit Raman lines which are polarized, the depolarization ratio pk can assume, according to Eqs. 2.4-11. .. 13 values of 0 < p < 6/7. All other Raman-active vibrations are emitting lines which are depolarized, they have a depolarization ratio of 6/7. The value 6/7 is appropriate for an arrangement, where the Raman radiation s investigated without an analyzer. If an analyzer is used 3/4 has to be taken instead. Cubic and icosahedral point groups are a special case the depolarization ratio for totally symmetric vibrations is 0. 

The XeFJ cation of C4 symmetry has the vibrational representation 3Raman active, but only 3oi + 3e are allowed in infrared absorption. Nine Raman lines attributable to fundamentals of XeF/ were observed, and assigned on the basis of their relative intensities, depolarization ratios, and by comparison with the spectra of related molecules (Table 4). The strong polarized lines at ca. 670 cm" and ca. 610... 

Strong polarized lines at 652 cm" and 585 cm" identify the [XeFsF J tetramer, and this is clearly the principal solute species in concentrated solution. The polarized Raman line at 621 cm" and the depolarized line at 508 cm" increase in intensity (relative to 652 cm" ) in more dilute solution their frequencies, relative intensities, and states of polarization correspond with the extra lines in hot liquid xenon hexafluoride, and, a little less closely, with the Raman spectrum of the vapour, (Table 8) which contains only XeFa molecules. We have little hesitation, therefore, in assigning these lines to a monomeric XeFj molecule. It is noteworthy that the... 

Our results add little to the question of the structure of ihe xenon hexafluoride molecule. The polarized Raman line al 621 cm could be assigned to the v, (a,) stretch of an octahedral molecule, and the depolarized line at 508 cm to V2 e,) the intensity pattern (v2 is comparable with i/ ) is very reminiscent of the Raman spectra of some isoelectronic molecules which are octahedral, viz. the hexahalogenotellurates(lV). Octahedral symmetry, however, cannot be reconciled with the complicated infrared spectrum of XeFs vapor, at least in the approximation of a simple harmonic force field. Pitzer and Bernstein have in any case provided persuasive evidence for XeFj monomer being substantially distorted in the Ti. bending mode from octahedral symmetry. This is in essential agreement with the bonding model proposed by Bartell and Gavin. ... 

P. Lallemand. Comparison of depolarized Raman and Rayleigh lines scattered by chloroform. Compt. Rend. Acad. Sci. Paris, ser, B, 72 429-432 (1971). 

B, Simic-Glavaski, D. A. Jackson, and J. G. Powles. Low frequency Raman lines in the depolarized wing of liquid toluene and benzene. Phys. Lett. A, 52 329-330 (1970). 

The symmetry property of a normal vibration can be determined by measuring the depolarization ratio. From an inspection of character tables (Appendix 1), it is obvious that d is nonzero only for totally symmetric vibrations. Then, Eq. 1.192 gives 0 < p < f, and the Raman lines are said to be polarized. For all nontotally symmetric vibrations, d is zero, and p = Then, the Raman lines are said to be depolarized. If the exciting line is plane polarized, these criteria must be changed according to... 

Calculate the depolarization ratio and indicate which Raman lines are polarized. 

The symmetry properties of resonance Raman lines can be predicted on the basis of Eq. 19.15. For totally symmetric vibrations, a and =0. Then Eq. 19.15 gives 0 sp <. Nontotally symmetric vibrations (q=0) are classified into two types those which have symmetric scattering tensors, and those which have antisymmetric scattering tensors. If the tensor is symmetric, = 0 and y, 9 0. Then Eq. 19.15 gives Py, = (depolarized). If the tensor is... 

The electric-field vector of a Raman line may correspond to partial or total polarization of the electromagnetic radiation in that line, depending upon the degree of symmetry of the molecular motion(s) that spawn the line. Thus, the symmetry of a particular vibrational mode may be inferred from the polarization (or depolarization) of a Raman band. 

The polarization of the light in a Raman line is obtained experimentally by measuring a quantity called the depolarization ratio, p. It is defined by a ratio of the band areas observed when the light is passed through a polarizer turned first perpendicular, then parallel to the scattered light. The expression that results is... 

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