Mechanism of Raman and Rayleigh scattering:

Let us know the mechanism of Raman and Rayleigh scattering along with the help of an energy level diagram. Figure provides a schematic energy level diagram of a molecule. The set of lines at the bottom end represent the electronic ground state and the corresponding vibrational energy levels. As same the set of lines at the top end represent the first electronic excited state and the equivalent vibrational energy levels. The region of continuum within the middle represents virtual states whose energies are not quantised.

Figure: Schematic representation of the energy changes associated with the excitation, Rayleigh scattering and Raman scattering

While a radiation, having a wavelength which is different from any of the absorption maxima of the molecule, interacts along with the molecule; the energy of the molecule increases through an amount equal to hν. The energy change could be represented through the upward arrow (black) to the far left in the figure. This suggests in which the molecule in the lowest vibrational state of the electronic ground state gets excited to a virtual state.