Depolarization ratio, Raman spectroscopy

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). 

Such considerations are of considerable importance in the study of oriented molecules, as in a crystal, but are less important for liquids and gases, in which the molecular orientation is random. However, in the case of Raman spectroscopy, not all information about the components of a is lost by orientational averaging. In particular one finds that it is possible to distinguish totally symmetric from other molecular vibrations from a measurement of the depolarization ratio,... 

This holds true especially for vibronically coupled systems. In absorption spectra, vibronic coupling effects are often hidden by Franck-Condon effects. Resonance Raman spectroscopy makes it possible to selectively enhance such hidden vibronic coupling effects by measuring the REPs of inducing modes, in particular those that are nontotally symmetric. A proper experimental study should include both the intensity and the depolarization ratio across the absorption band and should cover not only the fundamental but also overtones. 

M. Monan, J.-L. Bribes, R. Gauftes, Measurement of weak depolarization ratios of the Rayleigh scattering of gases using Raman spectroscopy. J. Raman Spectroscopy 12(2), 190-... 

PJ Treado, MD Morris. Infrared and Raman spectroscopic imaging. Appl Spectrosc 29 1-38, 1994. P Schmidt, PJ Hendra. The application of Fourier-transform Raman spectroscopy to the determination of conformation in poly( -caprolactam) chains. Spectrochim Acta 50A 1999-2004, 1994. SPS Porto. Angular dependence and depolarization ratio of the Raman effect. J Opt Soc Am 56 1585-1589, 1966. 

As already noted, whether a mode is IR or Raman active can be determined in advance (although its intensity cannot) by group theoretical considerations and depends on its symmetry type. For trans-l,2-dichloroethane (because ofits point group symmetry) each mode is eif/ier IR active or Raman active, but not both, and so IR and Raman spectroscopy excite completely different vibrational fundamentals. In the Raman spectrum, only bands of Ug and bg symmetry will appear. These can be distinguished experimentally by measuring the depolarization ratio according to theory (Wilson et al. 1955), the depolarization ratio oftotaUy symmetric modes ag under 2/1) is less than 0.75 (these are called polarized bands) whereas that of aU other modes is 0.75 (depolarized bands). (See the depolarization ratios calculated for trans-l,2-dichloroethane in O Fig. 10-5.)... 

FIGURE 3 Orientation of the E vectors of the incoming (E) and scattered light parallel (/ ) and perpendicular (/i) to the incident E vector. The depolarization ratio s p= l /l - [Reproduced from Carey, R R. (1982). Biochemical Applications of Raman and Resonance Raman Spectroscopies, Academic Press, New York, by permission.]... 

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