Scattered radiation :

The three components in the scattered radiation are observed only when the vibration causes a change in the polarisability. Here if the polarisability does not change in the course of vibration only Rayleigh scattering is observed. Therefore, we can conclude that in order to observe Stokes and anti-Stokes lines the polarisability of the bond must change in the course of vibration.

Let us raise a question. Since both the IR and Raman spectra include transitions amongst the same vibrational levels, would the two spectra be the similar.