Raman spectra depolarized

There is other information within the spectrum bandwidths and intensities in a Raman spectrum, depolarization ratios. 

The Raman spectrum can be used to give additional information regarding the symmetry properties of vibrations. This information derives from the measurement of the depolarization ratio p for each Raman band. The quantity p is a measure of the degree to which the polarization properties of the incident radiation may be changed after scattering... 

In Ref. (163) the techniques introduced above were illustrated for He-He and He-H2 collisional complexes. In Refs. (316,317) the property functions were applied in full quantum-statistical calculations of the dielectric second virial coefficient and of the polarized and depolarized Raman spectrum of the He gas. Some further applications were reported by Rizzo and collaborators318- 322. 

Scan the Stokes Raman spectrum of CCI4 for shifts of 150 to 1000 cm from the exciting line.t Note that the frequency in wavenumbers is given by v (cm ) = 1/A = vie, where c is the speed of light in cm s Record both parallel and perpendicular polarization scans so that you can determine the depolarization ratio of aU bands. Indicate the spectrometer wavelength or wavenumber reading on the chart at several points in the scan to provide reference points for the determination of the Raman shifts for all bands. 

Spectra of samples in the liquid state (Fig. 2.6-lB) are given by molecules which may have any orientation with respect to the beam of the spectrometer. Like in gases, flexible molecules in a liquid may assume any of the possible conformations. Some bands are broad, since they are the sum of spectra due to different complexes of interacting molecules. In the low frequency region spectra often show wings due to hindered translational and rotational motions of randomly oriented molecules in associates. These are analogous to the lattice vibrations in molecular crystals, which, however, give rise to sharp and well-defined bands. The depolarization ratio p of a Raman spectrum of molecules in the liquid state (Eqs. 2.4-11... 13) characterizes the symmetry of the vibrations, i.e., it allows to differ between totally symmetric and all other vibrations (see Sec. 2.7.3.4). 

A similar coupling may be observed for a, /J-unsaturated aldehydes and ketones. Table 4.1-3 shows that the s-cis or s-trans conformation of the C=C-C=0 systems is indicated by the difference of the frequencies of the C=0 and the C=C stretching vibration as well as by its relative intensity in the infrared and the Raman spectrum and the depolarization factor in the Raman spectrum (Oelichmann et al., 1982). 

The depolarization ratio of a band is very helpful in assigning vibrations in the Raman spectra of gases and liquids (Sec. 2.7.3.4). Vibrations of a totally symmetric representation can thus be distinguished from those associated with lower symmetry The breathing mode of the tetrahedral SiCU molecule at 424 cm , for instance, is the strongest band in one Raman spectrum but unobserved in another, depending upon the directions of the polarized radiation for excitation and observation. 

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... 

The Gas Phase Polarizability Anisotropy. Murphy50 has measured the depolarization ratio for Rayleigh scattering, pR, and analysed the intensity distribution in the rotational Raman spectrum of the vapour at 514.5 nm. The ratio R20 of the invariants of the a,-,aA/ tensor can be determined by fitting the rotational Raman distribution, and a is known (from the Zeiss-Meath formula). Knowledge of the three quantities, a, pR and R2o, allows the polarizability anisotropy, Aa, and the three principal values of the tensor to be calculated. The polarizability anisotropy invariant is numerically equal to the quantity,... 

The intensities of the spectral lines and the depolarization coefficients are functions of the reduced matrix elements of the polarizability tensor calculated by vibronic functions. In order to estimate the possibility of observation of the pure rotational Raman spectra under consideration, one has to consider in more detail the polarizability operator. Its components belonging to the line y of representation f can be presented in the form of a power series with respect to the displacement qriri active in the Jahn-Teller effect (the other components can be neglected as not active in the pure rotational Raman spectrum under consideration) ... 

Finally, M is introduced for convenience and represents the integral function of polarizability. It is the only function, the value of which depends on the polarizability characterizing the scattering medium. /iso( u) Raman spectrum depends only on the vibration motion and /aniso( y) depends on both vibration and reorientation motions of the molecule. In addition to the familiar qualitative use of the depolarization ratio, P = - Hv/. vv, to identify symmetric modes (i.e. p < 0.75), the quantitative studies of /iso and /aniso may provide detailed information about the vibration and reorientation relaxation. 

All the modes of coordinated nitrate groups are also active in the Raman spectrum, and it appears that the relative intensities of the Raman bands due to N—O stretches, together with their depolarization ratios, may prove to be a generally reliable criterion.31... 

What are the consequences of these considerations for depolarized light scattering In a dilute gas where reorientation is predominantly inertial, we expect the spectrum to be what is normally called the pure rotational Raman spectrum of the molecule. As higher densities are approached, the discrete spectral lines broaden and overlap to form a continuous band. We show how the band shape can be computed for freely rotating linear molecules and spherical top molecules and then indicate the assumptions that have been used by several authors to include collisions in the theory. 

The El modes, perpendicularly polarized in the IR, are depolarized in the Raman spectrum. Finally, the modes with the E2 symmetry are Raman-active and depolarized but inactive in the IR. Thus, with the use of vibrational spectroscopy and group theory, Koenig (9) deduced the helical structure of POE that consisted of a succession of trans, gauche, and trans forms around the O-C, C-C, and C-O bonds, respectively. These results were confirmed later 10, 11),... 

B2y, and >42 vibrations are expected to be polarized (p), depolarized (dp), and inversely polarized (tp ), respectively. These polarization properties, together with their vibrational frequencies, were used by Spiro and his coworkers to make complete assignments of vibrational spectra of the Fe-porphin skeletons of a series of heme proteins. They showed that the resonance Raman spectrum may be used to predict the oxidation and spin states of the Fe atom in heme proteins. For example, the Fe atom in oxyhemoglobin has been shown to be low-spin Fc(IIl). It should be noted that the A2y mode, which is normally Raman inactive, is observed under the resonance condition. Although the modes are rather weak in Fig. I-19, these vibrations are enhanced markedly and exclusively by the excitation near the B band since the A-term resonance is predominant under such condition. The majority of compounds studied thus far exhibit the A-term rather than the l -term resonance. A more complete study of resonance Raman spectra involves the observation of excitation profiles (Raman intensity plotted as a function of the exciting frequency for each mode), and the simulation of observed excitation proliles based on various theories of resonance Raman spectroscopy. ... 

The complementarity of IR and Raman spectra is based on the different excitation conditions change of dipole moment (vector quantity) in the case of an IR spectrum, change of polarization (tensor quantity) in the case of a Raman spectrum. Since a tensor is a three-dimensional quantity, the depolarization ratio p can be obtained by measuring Raman spectra with polarized Hght (polarization directions parallel and perpendicular to the optical plane ... 

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