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Why do Raman-scattered photons have higher or lower frequencies than the incoming light?
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spectroscopy
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Raman-scattered photons exhibit different frequencies than incoming light because of energy exchanges between the incident photons and the molecules they strike during the Raman effect.1
When a photon strikes a molecule, the interaction is typically elastic, resulting in scattered light with unchanged frequency. However, in some instances, the molecule and photon exchange energy:1
- Lower Frequency (Stokes lines): If a molecule in its ground state absorbs energy from the incident photon, the scattered photon loses energy. Because a photon's energy is proportional to its frequency, this loss results in a lower frequency and longer wavelength.12
- Higher Frequency (Anti-Stokes lines): If a molecule is already in an excited state, it may give up energy to the incident photon. The resulting scattered photon gains energy, leading to a higher frequency and shorter wavelength.12
These frequency shifts correspond to the energy transitions between the rotational and vibrational states of the scattering molecule, making them valuable for qualitative and quantitative analysis.1