Raman Spectroscopy Overview Web Text

Theory:  Raman Spectroscopy is a recently developed tool of the 20th century that provides insight into the molecular structure of materials.  By bathing a substance in quasi-monochromatic light, energy changes between the incident and emitted photons are observed as the molecule either gains or loses vibrational energy.  The gain of vibrational energy is known as a Stokes transition.  The loss of vibrational energy is known as Anti-stokes transition.  (Rayleigh occurs with the vibrational energy of the molecule is not changed) It is not difficult to realize the incredible utility of a tool like Raman Spectroscopy as it provides characteristics of any molecular compound.  A substance can be bathed in quasi-monochromatic light of a certain frequency and the resulting Raman emissions will show characteristics of its composition – molecular structure.  A main drawback to the Raman Effect is the difficulty in collecting the Raman emissions which are considerably less frequent than the Raleigh Effect.  Large samples were only able to be produced with a strong source, which was eventually realized with the development of high powered laser systems. Also, many different tools have been developed to sift through the emissions to collect only the Raman.  Different monochromators (i.e the filter) and combinations of monochromators have been used to filter extraneous emissions that blur the Raman emissions. 

Above, we have the energy states of a molecule and the corresponding transitions an electron would experience in one of these processes.  For Stokes, the energy of the incident photon makes the electron jump to a higher state, but the resulting emitted photon has less energy than the one incident so the electron drops down to an energy state slightly higher than before the absorption of the incident photon.  In Anti-Stokes, the process is exactly the opposite.  The incident photon has less energy than the resulting emitted photon.  Thus the electron loses energy which as discussed above is vibrational.

The Stimulated Raman Effect is what is now commonly used to observe the characteristic vibrational modes of certain molecules in a scattered beam (of different energy than the incident) produced by an incident quasimonochromatic beam.  This effect is produced through usage of high-energy laser pulses that occur in solids, liquids or dense gases.  The illustration below gives a general idea of what happens during the Raman Effect. [i]


[i]References:

Hecht, Eugene.  Optics.  San Francisco: Addison Wesley, 2002.

Ferraro, John R, and Kazuo Nakamoto.  Introductory Raman Spectroscopy.  San Diego, Academic Press, 

               1994.

Freeman, Stanely K.  Applications of Laser Spectroscopy. New York: John Wiley and Sons, 1974. 

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