What is the Difference Between Stokes and Anti-Stokes Lines?

Stokes and Anti-Stokes lines are spectral lines observed in Raman scattering, which is a non-destructive spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in molecules. The main differences between Stokes and Anti-Stokes lines are:

1. Energy Change: Stokes lines correspond to photons with lower energy compared to the incident photon, while Anti-Stokes lines correspond to photons with higher energy. In other words, Stokes lines are scattered photons that lose energy during the scattering process, and Anti-Stokes lines are scattered photons that gain energy during the process.
2. Transition Process: Stokes lines are generated through an inelastic scattering process, where the incident photons interact with a ground state molecule and form a virtual state. The scattered photons come from molecules that end up in excited vibrational states of the ground state. In contrast, Anti-Stokes lines occur when the molecules emit vibrational energy, resulting in an increase in photon energy.
3. Intensity: Stokes lines are generally more intense than Anti-Stokes lines due to the vibrational ground state being more populated than excited states.
4. Temperature Effect: Raising the temperature would decrease the population of the ground state and increase the population of higher energy vibrational states. As a result, the intensity of Stokes lines decreases while the intensity of Anti-Stokes lines increases.
5. Position in the Spectrum: Stokes lines can be observed towards the lower frequencies, while Anti-Stokes lines can be seen towards the higher frequencies. Later, they are mirrored at the center of the Raman spectrum.

In summary, Stokes lines are scattered photons with lower energy than the incident photon, and they are more intense than Anti-Stokes lines. Anti-Stokes lines are scattered photons with higher energy than the incident photon and are generally weaker than Stokes lines.

Comparative Table: Stokes vs Anti-Stokes Lines

The main difference between Stokes and Anti-Stokes lines lies in the wavelength and energy of the scattered radiation. Here is a table summarizing the differences between Stokes and Anti-Stokes lines:

In summary, Stokes lines represent scattered photons with longer wavelengths and lower energy than the incident photons, while Anti-Stokes lines represent scattered photons with shorter wavelengths and higher energy than the incident photons.