In 1928, Raman observed that when monochromatic radiation was incident on molecules, most of the scattered light had the same frequency, ν₀  of the incident light (Rayleigh Scattering) and in addition, there were some discrete frequencies above and below the frequency of the incident beam.  The process is refered to as Raman Scattering and the discrete lines are referred to as Stokes’ and anti-Stokes’ peaks.  The additional lines are mainly due to the rotational and vibrational levels of the molecules.

            You may recall from the earlier chapters that the rotational and vibrational spectra required the presence of the molecular dipole moment.  In Raman Spectroscopy, the transitions are induced by the fluctuating molecular polarizability.  Polarizability denotes the extent of possible distortion of the electron cloud in the presence of an external electric field.  The induced dipole moment is defined by

μ  = α E                                                                                                                                (15.9)

            Tightly bound electron clouds such as those in F₂ have low polarizability while soft and weakly bound molecules like I₂ have large values of polarizability.  Polarizability is a tensor.  The physical meaning of the polarizability tensor is that a field in one direction induces distortions not only along the field direction, but also along the other two directions.  Considering the 3 directions of the field (x,y,z), there will be nine components of polarizability which can be labeled as xx, xy, zz, yx, yy, yz, zx, zy, zz. Polarizability of nonspherically symmetric molecules is asymmetric.  The polarizability of a molecule is represented as an ellipsoid.  The polarizability ellipsoid of H₂ is shown in Fig. 15.6.