Polarized Raman lines

A few of the solutions showed weak polarized Raman lines at 920 em" and 570 cm" attributable to traces of XeOF these have been identified in the hgures. 

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. ... 

The in-plane vibrations (species A ) give polarized Raman lines. 

The spectra obtained are exactly analogous to those of binary mixtures of stannic halides (2), where analogous labile mixed molecules are formed. Each of the five species present is unmistakably characterised by an intense polarized Raman line of different frequency. Table I shows the close analogy between the chloro-bromo complexes of Ga " and the stannic chloro-bromides. 

On the other hand, the two Raman fundamentals of species Aig should yield polarized Raman lines, whereas the remaining five Raman lines are expected to be depolarized. Note that in this case the selection rules for fundamentals are mutually exclusive, that is, the infrared active fundamentals are Raman inactive, and vice versa. This is generally true of molecules possessing a center of invension, since the dipole moment is antisymmetric to such an operation, whereas the polarizability is symmetric. 

The observed Raman spectrum shows five lines including two polarized ones. Since polarized lines originate from totally symmetric normal modes, and the number of observed fundamentals exceeds those deduced for point group Tj and D411, the BU4 ion must have C2v symmetry. Note that this geometry is consistent with the prediction of VSEPR theory. The normal modes that give rise to polarized Raman lines are sketched below. 

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). 

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. 

Nonresonance Raman spectra of the alternating LB films were measured by a total reflection method shown in Figure 23. The films were deposited on quartz prisms. The s-polarized beam of 647.1 nm from a Kr laser was incident upon the interface between the quartz and film at an angle of 45° from the quarz side, and totally reflected. Raman line scattered from the film in the direction of 45° from the surface was measured through a Spex Triplemate by a Photometries PM512 CCD detector with 512x512 pixels operated at -125 °C. The spectral resolution was about 5 cm 1. 

The controllable polarization of the exciting laser light enables the polarization of Raman lines and their dependence on scattering angle to be studied... 

R.R Paradkar, S.S. Sakhalkar, X. He and M.S. Ellison, On-line estimation of molecular orientation in polypropylene fibers using polarized Raman spectroscopy, Appl. Spectrosc., 55, 534-539 (2001). 

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). 

Fig. 6.4. Typical CARS spectral profile modeled for an isolated Lorentzian Raman line assuming (6.5) (Tr = 4.6 cm-1, AT = 4cm 1) and parallel-polarized input and CARS fields. According to (6.10), the total CARS spectrum (solid line) is composed...

Fig. 11.5. Representative parallel-polarized (black lines) and cross-polarized (grey lines) Raman spectra of (A) sound and (B) carious enamel...

Fig. 14.3. (a) Bone section under polarized light, black line outlines where Raman images were acquired. Polarized Raman images of (b) phosphate V2/amide III, (c) phosphate Vi/amide I, and (d) carbonate/phosphate V2 band ratios at the interface between osteon and interstitial bone, (e and f) Three-dimensional view of phosphate Vi/amide I ratio for different polarization directions. Reprinted with permission from [1]... 

Paradkar, R.P. Sakhalkar, S.S. He, X. 8t Ellison, M.S. On-Line Estimation of Molecular Orientation in Polypropylene Fibers Using Polarized Raman Spectroscopy Appl. Spectrosc. 2001, 55, 534-539. 

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). 

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... 

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