Raman spectrum depolarization ratio

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... 

Beattie, I.R., Ozin, G.A., and Perry, R.O., Gas phase Raman spectra of P4, P2, As4 and As2. Resonance fluorescence spectrum of 80Se2. Resonance fluores-cence-Raman effects in the gas-phase spectra of sulfur and iodine. Effect of pressure on the depolarization ratios for iodine,. Chem. Soc., Perkin I, 2071, 1970. 

Besides being always Raman active, the totally symmetric vibrational modes can also be readily identified in the spectrum. As shown in Fig. 7.3.2, the scattered Raman radiation can be resolved into two intensity components, /l and 7. The ratio of these two intensities is called the depolarization ratio p ... 

One of the first VCD studies we undertook in 1988 was work on the tripeptide L-Ala-L-Ala-L-Ala [23], This molecule, in neutral aqueous (D O) solution, exhibits a distinct, near-conservative VCD spectrum in the amide I region, shown in Figure 7. The infrared absorption shows two overlapping peaks in the amide I region, at 1650 and 1675 cm 1. Raman depolarization ratios indicated that the high frequency component is the symmetric stretching combination. 

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). 

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). 

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. 

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. 

Figure 6.23. Raman spectra of liquid benzene observed with parallel and perpendicular polarization analyzers, 180° geometry (as in Fig. 6.22). Upper spectrum is parallel observation, middle is perpendicular. The lower spectrum is magnified by 10 to illustrate relative peak heights. Numbers indicate depolarization ratios, p =...

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,... 

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. 

A C n.m.r. study of the fluxional behaviour of the stereochemically non-rigid acetylenic carbonyl complex Co4(CO)ioC2(C6H5)2 has shown the exchange process to be a three-step and not two-step process as previously reported. Intensity and depolarization ratios have been measured from a Raman solution spectrum of Hg[Co(CO)4]2 and the carbonyl stretching force fields have been calculated. 

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... 

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 ... 

There is an indirect way to detect intermittent local collective motions. In the case of depolarized Raman scattering, the depolarization ratio is sensitive to low-frequency fluctuations in water. Depolarization is the scattering of the polarization of the electric field of light in a direction perpendicular to the original direction of polarization. Each fluctuating state has a distinct depolarization ratio. The intermittent character of the dynamics is known to appear as a so-called 1,/ frequency (f) dependence in a power spectrum. The power spectmm is obtained by Fourier transforming a time correlation function. 

Thus the Raman spectrum shows only the fundamental and the REP shows two peaks, corresponding to = 0 and v= 1. In the absence of degeneracies, the scattering tensor has no diagonal elements for nontotally symmetric modes, i.e., p depolarization ratio is then, according to Eq. (3),... 

Fig. 24. Raman excitation profiles and depolarization ratios for a trigonal Jahn-Teller ( X ej system. The absorption spectrum (not shown) is that of a displaced oscillator (see Figs. 3 and 4). The left graph represents linear (JC = I, J — 0), the right graph linear plus quadratic (K = 1, J = 0.2) coupling.

By polarizing the incident light or in other ways, it is possible to find the degree of depolarization of each frequency shift in the Raman spectrum, a quantity which will be important in the interpretation of the experimental results. This quantity is the ratio, for the scattered light, of the intensities of the components polarized perpendicular and parallel, respectively, to the direction of polarization of the incident illumination. 

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