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Stokes and Anti-Stokes Scattering

Stokes scattering leaves the molecule in a higher vibrational level, so the scattered photon is red-shifted; anti-Stokes starts from an already-populated vibrational level and returns a blue-shifted photon. Because the excited level population follows Boltzmann statistics, anti-Stokes lines are far weaker at room temperature, and their intensity ratio is a genuine thermometer.