Undergo vibration:

As you are aware, the molecules are never stationary and always undergo vibration (and may be rotation) motions. These motions may cause modifications in the polarisability of the molecule. For example, in the process of a vibration the bond length of a bond in the molecule remains varying and as the polarisability depends on the strength of the bond, it might modify the polarisability of the molecule. It could be shown (see the box given below) that while such a system vibrating along with a frequency of ν _vib interacts with the oscillatory electric field of the electromagnetic radiation then the scattered radiation might have three different components. One of these has frequency equivalent to the incident frequency (ν _ex) although the frequencies of the other two are equal to (ν _ex  +ν _vib ) and (ν _ex  -ν _vib ) respectively. The first component along with a frequency of ν _ex   is the reason for the Rayleigh scattering. The other two elements are the sources of the anti-Stokes and Stokes lines corresponding.