Question

Give a simple, laymen’s terms analogy to describe anti-stokes and stokes scattering.

Answer 1

Stokes scattering is when incident light of a given wavelength excites the molecule from the ground state to a virtual state and the molecule relaxes to a state which has higher energy than the ground state. This stokes scattering gives a wavelength which is longer than the incident wavelength of light (thus lesser energy than the incident photon), Rayleigh wavelength.

Anti-Stokes scattering is when incident light of a given wavelength excites the molecule from an already excited state to a virtual state and the molecule relaxes to the ground state. This stokes scattering gives a wavelength which is shorter than the incident (thus higher energy than incident photon), Rayleigh wavelength.

The frequency/wavelength shift of the scattered light from the incident light is indicative of the vibration frequency of the molecule and would change if the wavelength of incident light is altered.

This is the Quantum Mechanical picture. The classical picture of Raman scattering (Stokes and Anti-Stokes scattering) can be imagined through an analogy (however it has many drawbacks and should hence not be taken seriously at its entirety):

Imagine a ball striking a plate mounted on an oscillating spring. If the ball strikes out of phase to the oscillating plate when the plate is just about to come to rest at its maximum displacement, the recoil of the ball will be small and the ball will not have enough energy after reflection. Here, the ball is analogous to a photon and it implies that the ball now has less energy (larger wavelength) which paints the picture of Stokes scattering.

Similarly, when the ball strikes the plate at the right time when the plate just passes through its center and has maximum velocity, the recoil velocity of the ball will also be maximum and it will thus possess a higher energy after reflection(analogous to shorter wavelength) thereby describing Anti-Stokes scattering.