Intensity and Polarization of Raman Lines

Intensity of Raman Lines:

The intensity of a Raman Line when expressed as a function of the parent line is usually a few hundredths in liquids and few thousands in gases. In spite of the practical difficulties in measuring the intensities of Raman lines results of fundamental importance have been obtained. The variations in intensity as we pass from line to line and from substance to substance are of great significance in the study of molecular structure and chemical constitution. The Stokes lines are always more intense than the corresponding anti-Stokes lines; these, however, grow more intense as the temperature is raised. All the Raman lines move inward towards the parent line with an increase in temperature.

Polarization of Raman Lines:

Just as the Raman Lines vary greatly in their intensities, so also their states of polarization. The fact that different lines are differently polarized is probably connected with their relative intensity.

The experimental arrangement used for determining the polarization of Raman Lines is essentially the same as that used in the experimental study of the Raman Effect but with the following modifications. The light from the source is concentrated by means of a condenser into the substance contained in the Raman tube. A suitably oriented double image prism whose function is to separate the vertical and horizontal components in the scattered light is placed in front of the slit of the spectrograph, so that two images, one above the other are formed on the slit, which are simultaneously photographed.

The state of polarization of a Raman line is measured by a quantity known as the depolarization factor which is simply, the ratio of the intensities of the horizontal and vertical components when the incident light is vertically polarized. The ratio is readily obtained from the traces photographed as described above, by one of the usual methods employed for comparing the intensities of two beams of the same wavelength. In order to get fairly accurate values of the depolarization factor, the following precautions should be taken:

(1) Crystalline quartz should not be used for condensers, spectrographs, or windows, since its optical activity complicates the phenomenon of polarization.

(2) The window of the Raman tube through which the scattered light emerges should be stain-free and plane.

(3) Errors arising from oblique refraction at the prism surfaces, want of transversality in the incident beam, and slit width should be eliminated.

Cabannes found that the Raman Lines in crystals, such as quartz, are differently polarized, and the intensity and depolarization of the lines depend upon the orientation of the crystal. Menzies has investigated the polarization of the Raman lines in liquids, such as CCl4, in directions perpendicular and obliquely forward to the incident beam and has shown that many of the observed facts could be accounted for by considering the initial and final directions of vibrations in the molecule involved to be parallel in the case of polarized lines, perpendicular in that of unpolarized lines and at an oblique angle for partially polarized lines. The following are some of the important results obtained:

(1) The depolarization factor varies from 0 to 0.86 for the vibrational Raman lines, while it has a constant value of 0.86 for the rotational lines.

(2) With circularly polarized incident light, part of the Ramn Line is circulalrly polarized in the reverse direction and part circulalrly polarized in the same direction as that of the incident light. Highly depolarized rotational lines exhibit reverse circular polarization.

(3) Corresponding line in molecules having similar structures have nearly the same depolarization factors. This is to be expected as the polarization of a Raman line is decided by the symmetry of the oscillation.

Solid Solution
Vapour Pressure and Raoult’s Law
Ideal and Non-Ideal Solutions
Azeotropic Mixtures
Colligative Properties
Relative Lowering in Vapour Pressure
Elevation of Boiling Point
Depression in Freezing Point
Nuclear Physics– Tamil Board

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