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Asked in GSEB Board July 2015 · Dispersion and scattering
Given: the Stokes and anti-Stokes lines of a Raman spectrum.
Idea: for any scattering event, ask whether the photon keeps its energy.
Most scattered photons do keep it exactly; that is elastic (Rayleigh) scattering, and it gives the strong line at the incident frequency.
A small fraction exchange energy with the molecule. A photon that hands energy over emerges with less and appears below the incident frequency -- a Stokes line; one that collects energy from an already excited molecule emerges with more -- an anti-Stokes line.
An exchange of energy is precisely what makes a collision inelastic, and the size of each shift measures one of the molecule's own vibrational energy steps.
So the Stokes and anti-Stokes lines are due to inelastic scattering of light.
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