Depolarisation Ratio:

You know that the Raman scattered light is a result of the interaction of the electric field of the incident light with the scattering species. Further, the laser used for the excitation are polarised, that is, the electric field of the radiation vibrates in a specific direction. It is expected in which the scattered light would also be same polarised. However, in actual practice some of the scattered light is found to be depolarised to varied extents. This will depends on the symmetry of the molecular vibration causing the scattering and is a result of the nature of the polarisability changes during the vibration. Let us try to understand it.

The polarisability of the scattered light is measured through passing it through a Nicol prism or a polaroid film called analyser. An intensity of the light passing by the analyser is measured as the analyser is rotated. Intensity would be maximum while the analyser is parallel to the plane of maximum polarisation and would decrease as the analyser is rotated. That will be minimum while the analyser is perpendicular to the plane of maximum polarisation. To the convenience of expression, a nature of the scattered light is expressed in terms of degree of depolarisation rather the degree of polarisation. The degree of depolarisation, ρ, is described as

ρ = I_⊥/I _{I I}

where

 I_⊥ and I _{I I}   are the minimum and the maximum intensity measured by the analyser.

The degree of depolarisation helps within the assignment of the signals in the Raman spectrum. In common, a symmetric vibration provides a polarised or partially polarised Raman line whereas the nonsymmetric vibrations give depolarised signals. As per to scattering theory estimates a minimum degree of depolarisation for a nonsymmetric vibration is 6/7 and it is termed as depolarised. On the other hand the degree of depolarisation for the symmetric vibrations is always less than 6/7 and the vibration is called polarised.