The polarizability of an electron cloud changes when it undergoes vibration.When a beam of radiaion is incident on such molecules undergoing polarizability, it scatters.These scattered radiations may be parallel or perpendicular to the plane of polarization of the incident radiations having intensities represented I ll and I┴ respectively.The degree of polarization called depolarization ratio is (ρn) given by the relation:

ρn = I┴/ I ll

When the polarizability molecule does not change during vibration, the ρn is zero but when the polariziability changes then (ρn) is non zero.

A line is classified as as Depolarized Line if the value of (ρn) is more than 0.75 while a line with (ρn) value less than 0.75 is called Polarized Line.

Totally symmetric vibrations cannot lead to depolarization.

We know that for a radiation to undergo Raman transition-its electronic polarizability must change and ∆ v = ±1

The study of depolarization of lines helps in the assignment of vibrations in molecular structure and simples its interpretation.

Group theory assigns each molecule a definite point group like H2O(C2v), BF3(D3h). This is based on symmetry elements(E, α, i,Cn, Sn) having the corresponding symmetry operation( identity, reflection, inversion, rotation axis and rotation- reflection axis respectively). Then follow the steps like- Assign point group, Calculate vibration degrees of freedom, Work out Chacacters for each reducible representations under each symmetries, Calulate Ttotal, work out value of ai for each irreducible representations to give Tvib, calculate Tbonds and so on--etc. From these operations, we construct the Character Table of each point gtoup.A character Table possess Irreducible Representations which is made up of some or all these elements like E, sigma, I,Cn, Sn. They give rise to Vibration Symmetry Classes like A1. A2, B1, B2 ---- and many more .

The Raman sectrum of these Vibration Symmetry Classes belonging to a compound contain a number of vibrations represented by two parmete- the r(n) and overall Raman intensities and classified as:

[i] (ρn) =0.75 and a high values of overall Raman intensities

[ii] ρn =0.75 and small values of overall Raman intensities

[iii] (ρn) =0.75 and a zero value of overall Raman intensities

[iv] ρn ˂0.75 and large values of overall Raman intensities

[v] ρn ˂0.75 and small values of overall Raman intensities

[vi] ρn ˂0.75 and zero values of overall Raman intensities

Cases[i] , [ii] and [iv] are expected to be observed.

[i] and [ii] cases most probable to be observed.

And thus the Raman spectra are simplified.

Thanks Manohar,

I want to also know that exciting radiation is itself polarised or it get polarised after scattering from molecule. and if we are using polariser, which component of exciting radiation (magnetic or electric component) stucking molecule to get Raman line?

It is the polarizability  of the molecule which polarizes the incident radiation

 Pls  read -When a substance is irradiated with a monochromatic light of definite frequency

For measuring depolarization ratio, samples are excited by plane polarized light from a laser source.

If it is so , I had a wrong notion as on comparison of IR and Raman, the use the sentence- ----the analogous conditions----- is quite common and thus the wrong concept. .

For respected Drs. Shivaji Tayade and V. N. Ravi Kishore V.

As an academic discussion is the essence of understand ing the finer points of any topic, I humbly attach a file which lay stress on three points.

PLS. SEE:Slides-1/II

[A] The celebrated Dr Raman used simple sunlight in his original experiment  and not the ppl [ first para]

[B] The use of higher energy LASER was an an improvement because only one out of 10^7 photons would scatter and LASER would greatly enhance the out put signal [ second para]

[C]It is molecular vibrations which change the polarizability.

PLS SEE :Slide-III[ A line written in bold]

RAMAN SCATTERING OCCURS BECAUSE A MOLECULAR VIBRATION CAN CHANGE THE POLARIZABILITY.

Respected Dr. Manohar Sehgal,

I salute your interest, dedication and effort even after retirement.

I have something to say

Raman effect or Raman scattering can be seen even with unpolarized light. But depolarization ratio measurement experiment requires a polarized light source.

In this experiment, one is looking at how scattering from a molecule depolarizes light.

 For Dr.V. N. Ravi Kishore V. Thks. For me it  a way to spend my retirement among-st young scholars like your self.I simply wanted to convey that:

It is molecular vibrations which change the polarizability .Even if there is no ppl, there would be a change in polarizability.

Polarizability is a property of the molecule.

If one uses unpolarized light, it would be difficult to measure depolarization ratio.

Unpolarized light can be treated as combination of light of different polarizations and there would be depolarization associated with each ppl.

Sir,

Did Dr. Raman use PPL in his original experiment? If yes- then   I had been carrying a wrong notion  throughout my teaching career.If no- I will   feel relieved that I was not wrong.Either way, the discussion will triumph.

Dr. Raman did not use PPL in his experiment.

But Dr. Raman also did not measure depolarization ratio.

The nature of Raman spectra may depends on the incident and scattered polarisation of the wave ... 

you can find the Raman spectra as well as the corresponding spectral peak intensity distribution for the different facets of AlGaN hexagonal sub-microrod from the following article....

https://www.researchgate.net/publication/290123296_Polarized_Raman_and_photoluminescence_studies_of_a_sub-micron_sized_hexagonal_AlGaN_crystallite_for_structural_and_optical_properties

thank you......!

Article Polarized Raman and photoluminescence studies of a sub-micro...

The polarization of the Raman scattered light depends on the symmetry of the molecular vibration responsible for the Raman line in question. See Herzberg for details.