Raman Spectroscopy

Raman spectroscopy is often considered to be complementary to IR spectroscopy.  For symmetrical molecules with a center of inversion, Raman and IR are mutually exclusive.  In other words, bonds that are IR-active will not be Raman-active and vice versa.  Other molecules may have bonds that are either Raman-active, IR-active, neither or both.

Raman spectroscopy measures the scattering of light by matter.  The light source used in Raman spectroscopy is a laser.  The laser light is used because it is a very intense beam of nearly monochromatic light that can interact with sample molecules.  When matter absorbs light, the internal energy of the matter is changed in some way.  Since this site is focused on the complementary nature of IR and Raman, the infrared region will be discussed.  Infrared radiation causes molecules to undergo changes in their vibrational and rotational motion.  When the radiation is absorbed, a molecule jumps to a higher vibrational or rotational energy level.  When the molecule relaxes back to a lower energy level, radiation is emitted.  Most often the emitted radiation is of the same frequency as the incident light.  Since the radiation was absorbed and then emitted, it will likely travel in a different direction from which it came. This is called Rayleigh scattering.  Sometimes, however, the scattered (emitted) light is of a slightly different frequency than the incident light.  This effect was first noted by Chandrasekhara Venkata Raman who won the Nobel Prize for this discovery. (6)  The effect, named for its discoverer, is called the Raman effect, or Raman scattering.  

Raman scattering occurs in two ways.  If the emitted radiation is of lower frequency than the incident radiation, then it is called Stokes scattering.  If it is of higher frequency, then it is called anti-Stokes scattering.

Energy Diagram Scattering The green arrow in the picture to the left represents the incident radiation.  The Stokes scattered light has a frequency lower than that of the original light because the molecule did not relax all the way back to the original ground state.  The anti-Stokes scattered light has a higher frequency than the original because it started in an excited energy level but relaxed back to the ground state.
Though any Raman scattering is very low in intensity, the Stokes scattered radiation is more intense than the anti-Stokes scattered radiation. The reason for this is that very few molecules would exist in the excited level as compared to the ground state before the absorption of radiation.  The diagram shown represents electronic energy levels as shown by the labels "n=".  The same phenomenon, however, applies to radiation in any of the regions.

SERS In the infrared region, methods have been developed to enhance the intensity of the scattered light in order to make Raman instruments more sensitive.  One of these methods involves placing the sample on roughened gold or silver surfaces, then focusing the light on the sample.  The laser light is chosen such that it will interact with the surface particles in order to resonate, therefore increasing the intensity of the scattered light.  This greatly enhances the ability to detect the scattered radiation.  (7) This is called Surface Enhanced Raman Scattering or SERS.  SERS has made it much more practical to detect the scattered light, making Raman a more available technique.                                                                                                 

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