From Rayleigh to Raman

Rayleigh Scattering is the reason why the sky is Blue, Mars is Red but the martian sunsets are Blue and also why the color of the smoke from a cigarette is colorless, but the puffed out smoke is White and so on.

But, Rayleigh scattering is the elastic scattering of light i.e the energy( and therefore, the wavelength) of the incident photon is conserved and only its direction is changed. You can consider the Newton’s cradle as an analogy.

But this is not the only mode of interaction that can exist. Inelastic collisions are also a possibility. This likelihood brings us face to face with Raman Scattering.

Principle

In Raman Scattering, when light is incident on a molecule, the molecules gets excited. As a result of this excitation, the emergent frequency of light is different that of the Incident light.
Awesome, what can I do with that?

If one can extract all of the vibrational information corresponding a molecule, its molecular structure can then be determined.

For polarizable molecules, the incident photon energy can excite vibrational modes of the molecules, yielding scattered photons which are diminished in energy by the amount of the vibrational transition energies.






The vibrational modes for Water and Carbon Di-oxide.

“Fingerprinting” the moon

The spectrum of the Raman-scattered light depends on the molecular constituents present and their state, allowing the spectrum to be used for material identification and analysis.

Also,since the Raman spectra can be collected remotely, they show great promise for planetary exploration.