PET-PMT

[Geek_ji]

Short Answer:
A process by which light interacts with vibrating atoms in a material, resulting in the transfer of energy to different wavelengths.

Simple Answer:
You are welcome to post a simple answer to this questions.
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[Nerd_ji]

Here is a more elaborate explanation. It may not be too simple. If you have a simpler explanation you are welcome.

The Raman effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The incident photon excites one of the electrons into a virtual state. For the spontaneous Raman effect, the molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, which generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-Stokes Raman scattering.