Polarizability components determine Raman activity

Symmetry selection rules for Raman spectrum can be derived by using a procedure similar to that for the IR spectrum. One should note, however, that the symmetry property of symmetry species of six components of polarizability are readily found in character tables. In point group C3V, for example, normal vibrations of the NH3 molecule (2A1 and 2E) are Raman-active. More generally, the vibration is Raman-active if the component(s) of the polarizability belong(s) to the same symmetry species as that of the vibration. 

Stokes (o)q+(0 ) scattering processes. The partial derivative factor, (3aij/9Qk)e evaluated at the equilibrium position of the normal coordinate comprises a necessary condition for Raman activity of the normal mode Q. Raman effects occur only for those normal modes that cause the molecule to undergo a net change in polarizability during the course of the vibration. While equation (7) implies that both Stokes and anti-Stokes components should appear with equal intensity, a quantum mechanical derivation shows that the Stokes/ anti-Stokes intensity ratio is proporti onal to the Boltzmann factor (7), and can be used to determine the molecular temperature of a collection of molecules. The statistical derivation is based upon the thermal population of ground and excited molecular vibrational states according to a Boltzmann distribution. 

After obtaining the symmetry point group of the metal-adsorbate system, one can determine which symmetry representations of the vibrations of the molecule will be Raman active using group character tables for the given point group. Raman-active modes are those that possess nonzero components of the Raman polarizability tensor. 

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