Raman scattering intensity ratio, change

FIGURE 1.8 Change of electrical conductivity against Raman scattering intensity ratio I260l I640 at 260 cnr1 and at 640 cm-1 [29]. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Raman and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. 

Stokes (o)q+(0 ) scattering processes. The partial derivative factor, (3aij/9Qk)e evaluated at the equilibrium position of the normal coordinate comprises a necessary condition for Raman activity of the normal mode Q. Raman effects occur only for those normal modes that cause the molecule to undergo a net change in polarizability during the course of the vibration. While equation (7) implies that both Stokes and anti-Stokes components should appear with equal intensity, a quantum mechanical derivation shows that the Stokes/ anti-Stokes intensity ratio is proporti onal to the Boltzmann factor (7), and can be used to determine the molecular temperature of a collection of molecules. The statistical derivation is based upon the thermal population of ground and excited molecular vibrational states according to a Boltzmann distribution. 

Bulkin et al. [48] showed that the intensity ratios 1096/1117, 1452/1460, and 1415/ 1409 correlated nearly linearly with the 1730-cm FWHM. However, when the orientation of the samples differs substantially, the 1730-cm FWHM does not correlate exactly with density. They also found that the intensity ratio 795/792 varied nearly linearly with the wide-angle x-ray scattering measurement of the crystallinity. However, this ratio did not correlate as well with the 1730-cm" FWHM. They interpret this as indicating that the 795/792 ratio is an indicator of the true crystallinity, whereas the other bands only indicate conformational changes that occur both due to orientation and to crystallization. Figure 7 shows the Raman spectrum over the range 100-1900 cm for a high- and a low-crystallinity fiber. 

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