What is polarised Raman spectroscopy?

Polarised Raman spectroscopy probes information about molecular orientation and symmetry of the bond vibrations, in addition to the general chemical identification which ‘normal’ Raman provides.

Polarised Raman measurements are made acquiring spectra with polarisation which is either parallel or perpendicular to the inherent polarisation of the excitation laser.  The measurement is made by inserting a polarisor in the beam path between the sample and the spectrometer, allowing the Raman polarisation to be selected by the user.  The polarisation of the laser beam can also be kept in its normal state, rotated by 90o, or ‘scrambled’ to remove any polarisation by inserting polarising optics between the laser and the sample.

Polarisation measurements provide useful information about molecular shape and the orientation of molecules in ordered materials, such as crystals, polymers and liquid crystals.

An example of the use of polarised Raman spectroscopy is to characterise the symmetry of bond vibrations in a molecule.  This is done by calculating the depolarisation, ρ, for a particular peak, where

ρ= ι / ιII

and ι is the intensity of the Raman band with polarisation perpendicular to the laser beam, and ιII is the intensity with polarisation parallel to the laser beam

If ρ<0.75 the vibration can be considered to be polarised, and is totally symmetric in nature.  If r=0.75 then the vibration can be considered to be depolarised, and is non-symmetric in nature.

 

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