Editing Raman spectroscopy (section)

=== Characterization of the symmetry of a vibrational mode ===
The polarization technique is useful in understanding the connections between [[molecular symmetry]], Raman activity, and peaks in the corresponding Raman spectra.<ref>{{cite journal|last1=Itoh|first1=Yuki|last2=Hasegawa|first2=Takeshi|title=Polarization Dependence of Raman Scattering from a Thin Film Involving Optical Anisotropy Theorized for Molecular Orientation Analysis| journal=The Journal of Physical Chemistry A |date=May 2, 2012|doi=10.1021/jp301070a| pmid=22551093|volume=116|issue=23 | pages=5560–5570| bibcode=2012JPCA..116.5560I}}</ref> Polarized light in one direction only gives access to some Raman–active modes, but rotating the polarization gives access to other modes. Each mode is separated according to its symmetry.<ref>{{cite journal|display-authors=4| last1=Iliev|first1=M. N.| last2=Abrashev|first2=M. V.| last3=Laverdiere|first3=J.| last4=Jandi|first4=S.| last5=Gispadinov|first5=M.M.| last6=Wang|first6=QY.-Q| last7=Sun|first7=Y.-Y| title=Distortion-dependent Raman spectra and mode mixing in RMnO<sub>3</sub> perovskites (R=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Y)| journal=Physical Review B | volume=73| issue=6| pages=064302|date=February 16, 2006 |doi=10.1103/physrevb.73.064302 |bibcode=2006PhRvB..73f4302I |s2cid=117290748}}</ref>

The symmetry of a vibrational mode is deduced from the depolarization ratio ρ, which is the ratio of the Raman scattering with polarization orthogonal to the incident laser and the Raman scattering with the same polarization as the incident laser: <math>\rho = \frac{I_r}{I_u}</math> Here <math>I_r</math> is the intensity of Raman scattering when the analyzer is rotated 90 degrees with respect to the incident light's polarization axis, and <math>I_u</math> the intensity of Raman scattering when the analyzer is aligned with the polarization of the incident laser.<ref name=Banwell>{{cite book |last1=Banwell |first1=Colin N. |last2=McCash |first2=Elaine M. |date=1994 |title=Fundamentals of Molecular Spectroscopy |url=https://archive.org/details/isbn_9780077079765 |url-access=registration |edition=4th |publisher=McGraw–Hill |pages=[https://archive.org/details/isbn_9780077079765/page/117 117]–8 |isbn=978-0-07-707976-5 }}</ref> When polarized light interacts with a molecule, it distorts the molecule which induces an equal and opposite effect in the plane-wave, causing it to be rotated by the difference between the orientation of the molecule and the angle of polarization of the light wave. If <math display="inline">\rho \geq \frac{3}{4}</math>, then the vibrations at that frequency are ''depolarized''; meaning they are not totally symmetric.<ref>{{cite web|url=http://www.horiba.com/us/en/scientific/products/Raman-spectroscopy/Raman-academy/Raman-faqs/what-is-polarised-Raman-spectroscopy/|title=What is polarised Raman spectroscopy? - HORIBA| website=horiba.com}}</ref><ref name=Banwell/>