What is Raman Spectroscopy

Raman spectrum of a silicon crystal showing, from left to right, anti-Stokes, Rayleigh, and Stokes lines. The frequency scale is expressed as the Raman shift with respect to the excitation wavelength this is why the Rayleigh scattering is at 0cm  

Raman lines corresponding to the optical phonons can be seen symmetrically shifted from the incident laser line. For ease of eomparison to IR speetroscopy, frequency shifts are expressed in wavenumbers (cm ). The Stokes shift is most commonly measured at room temperatures, as from simple thermodynamics using Boltzmann s equation, there are very few vibrations in most materials at room temperature which can contribute to anti-Stokes scattering. 

At room temperature, the number of molecules in an excited vibrational state will be low. This can be shown by using Boltzmann s equation  

In a simplified diatomic molecule, in the harmonic oscillator approximation, as in the case for IR spectroscopy, the frequency of vibration is given by  

Since its discovery in 1928, Raman spectroscopy has evolved in terms of the fundamental understanding of the process, instrumentation and applications. More advanced techniques such as Resonant Raman Spectroscopy (RRS)  

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Figure 2.18 The infrared and Raman spectra of styrene/butadiene rubber illustrating the complementary nature of the two techniques (From What is Raman Spectroscopy , Hendra, P., Int J Vib Spect, [www.ijvs.com] 1 (5), pp. 6-16 (1998) f Copyright John Wiley Sons Limited. Reproduced with permission).

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