Scattering depolarizing mode

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 ... 

The processes observed in the depolarized Rayleigh spectrum correspond to internal modes of motion. Thus, they may have relaxation times which substantially exceed those obtained from the longitudinal or bulk relaxation alone. Nevertheless they are a part of the a relaxation process as it is normally observed in the creep compliance. All processes with the same shift factors make up the full a relaxation. In liquids with substantial depolarized Rayleigh scattering the slowly relaxing part of the W scattering is also dominated by the orientation fluctuations associated with the internal modes of motion. Each internal mode contributes some intensity, but it is believed that fairly short wavelength modes dominate the scattered intensity. 

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 ... 

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. 

Computer programs such as Gaussian 03 can simultaneously calculate several other parameters including normal modes (shown in Table 1.15), IR intensities, Raman scattering intensities, and depolarization ratios. 

Additional information about the vibration can be obtained through the depolarization ratio. This is the ratio of the intensity of scattered light that is polarized in a plane perpendicular to the incident radiation relative to that the scattered light that is polarized parallel to the incident polarization, p = For totally symmetric modes, p = 0, while 0 < p < 3/4 for non-totally symmetric modes [i, 3]. The polarization ratio can actually be greater than 3/4 for a resonantly enhanced Raman band [3]. 

The depolarized scattering for the Rouse-Zimm dynamical model of flexible polymer chains (cf. Section 8.8) may also be calculated. Ono and Okano (1971) have performed this calculation for q = 0 (zero scattering angle) and find that the scattered light spectral density is a series of Lorentzians each with a relaxation time characteristic of one of the Rouse-Zimm model modes. However the contribution of each mode to the spectrum is equal. This behavior should be contrasted with that of the isotropic spectrum where the scattering spectrum is dominated by contributions from the longest wavelength modes. 

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