Stokes and Anti-Stokes Lines
Definition
Stokes lines (longer wavelength, lower energy than the laser) come from molecules starting in the ground vibrational state; anti-Stokes lines (shorter wavelength, higher energy) come from molecules already in an excited vibrational state. Stokes lines are much more intense at room temperature.
- Stokes lines
- Longer wavelength, lower energy than the laser
- Anti-Stokes lines
- Shorter wavelength, higher energy than the laser
- Origin
- Molecular vibrational state before scattering
- Relative intensity
- Stokes lines much more intense at room temperature
Common questions
Why are Stokes lines used for routine Raman spectroscopy instead of anti-Stokes lines?+
At room temperature most molecules start in the ground vibrational state, which is the population that produces Stokes scattering, while far fewer molecules are already in an excited vibrational state to produce anti-Stokes scattering, making Stokes signal much stronger and easier to measure reliably.
Does the ratio of Stokes to anti-Stokes intensity carry any forensic information?+
Yes, because the anti-Stokes population depends on thermal excitation, the Stokes to anti-Stokes intensity ratio can be used to estimate the local sample temperature, though this application is specialised and not part of routine identification work.
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