Raman Spectroscopy:
3. A photon with frequency v0 is absorbed by a Raman active molecules, which is already in its excited vibrational state. As a result, excess energy of excited Raman active mode is released and hence molecules returns to the basic vibrational state resulting in a frequency of scattered light goes up to v0 + vm. This is called as Anti-Stokes frequency or simply Anti-stokes.
4. In most of the general cases, main part of the incident light is transmitted without interaction, which is quite useless for practical purposes of molecular characterization.
A part of the incident light may also be absorbed by allowed electronic transitions and is emitted as → Fluorescence.
Only about 10-3 % of the incident light produces inelastic scattering of Raman signal with frequencies v0 ± vm. Hence, special measures are done to distinguish the Raman signal from the dominant Rayleigh scattering by adopting various tools such as filters, apertures, double and triple spectrometric systems.