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I've seen an off-hand reference to Raman being a Lambertian source and it's been tripping me up. My understanding is that on the molecular level the laser is exciting the vibrational mode of a bond which results in a higher virtual electron state. Within a few picoseconds the vibrational mode decay emits a Stokes photon, but why wouldn't that just be in a random direction? If it is a Lambertian source, is this due to the stimulated emission of photons in the path of the laser in the sample effectively creating gain? A Raman signal from a 2d target might make sense as a Lambertian source due to physical geometry but in a liquid sample?

This goes a bit out of my comfort zone but my understanding of the Raman effect is that it is a generalization of the Rayleigh scattering process by adding a polarization term which depends on the vibration of bonds, resulting in an heterodyne process between the photon frequency and the vibration frequency. I would therefore expect the emission distribution to be the same as in the Rayleigh scattering process:



The distribution is close to a lambertian pattern, reason for which, I guess, the reference you mentioned is talking about Raman being a lambertian source.

Again, my words are to be taken with caution as it's reaching the limit of my current understanding of the physics behind it.