Depolarization ratio

For other uses, see Depolarization (disambiguation).

In Raman spectroscopy, the depolarization ratio is the intensity ratio between the perpendicular component and the parallel component of the Raman scattered light.^[1]

Early work in this field was carried out by George Placzek, who developed the theoretical treatment of bond polarizability^[2]

The Raman scattered light is emitted by the stimulation of the electric field of the incident light. Therefore, the direction of the vibration of the electric field, or polarization direction, of the scattered light might be expected to be the same as that of the incident light. In reality, however, some fraction of the Raman scattered light has a polarization direction that is perpendicular to that of the incident light. This component is called the perpendicular component. Naturally, the component of the Raman scattered light whose polarization direction is parallel to that of the incident light is called the parallel component, and the Raman scattered light consists of the parallel component and the perpendicular component. The ratio between these two components is expressed as the depolarization ratio.

The value of the depolarization ratio of a Raman band depends on the symmetry of the molecule and the normal vibrational mode, in other words, the point group of the molecule and its irreducible representation to which the normal mode belongs. Under Placzek’s polarizability approximation, it is known that the depolarization ratio of a totally symmetric vibrational mode is less than 0.75, and that of the other modes equals 0.75. A Raman band whose depolarization ratio is less than 0.75 is called a polarized band, and a band with a 0.75 depolarization ratio is called a depolarized band.

The ratio of the peak intensity of the parallel and perpendicular component is known as the depolarisation ratio. And can be obtained as shown in equation 1.

$\rho = \frac{ I_{perpendicular} }{ I_{parallel} } \quad\textrm{or}\quad \frac{ I_{depolarized} }{ I_{polarized} }$

^[3]

Equation 1. Raman depolarisation ratio. Where 'I_{perpendicular}' signifies the peak intensity with excitation and Raman analysis polarisers orientated perpendicular to one another.

For example if you have a species with a peak which had an intensity 10 units, when the polarisers were parallel, and an intensity of 1 unit when the polarisers were perpendicular, then the depolarisation ratio would be 1/10 = 0.1, this peak would be highly polarised band.

Spectral Images shown here are taken, with permission from PerkinElmer Raman Division.

References

1. ^ Charly D. Allemand, "Depolarization Ratio Measurements in Raman Spectrometry", Applied Spectroscopy 24(3), 1970, pp. 348–353
2. ^ D. A. Long (Apr. 8, 1953). "Intensities in Raman Spectra. I. A Bond Polarizability Theory". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 217 (1129): 203–221. JSTOR 99022. 
3. ^ PerkinElmer Technical Note, "Raman Polarization Accessory for the RamanStation 400", http://las.perkinelmer.com/content/relatedmaterials/productnotes/PRD_RamanPolarizationAccessory4RamanStation400.pdf