Thomson - Rayleigh Scattering: Rotational & Vibrational Energy Levels

• on the far-infrared and microwave region the molecule gets rotational energy.(microwave spectra)
• on the near-infrared region the molecule gets both rotational and vibrational energy.(infrared spectra)
• on the ultraviolet & visible light region, the molecule gets all energy levels rotational, vibrational, and electronic energy (electronic band spectra)

• Rayleigh Scatter does not excite electrons of atoms to another electronic level but only causes them to oscillate.
• Thomson Scatter happens when an electron is free or quasi-free and it also makes the electron oscillate and more, accelerate.

• Thomson Scattering can not cause rotational spectra or energy build-up, which requires the whole molecule to rotate since Thomson Scattering is about free and quasi-free electrons. But what about vibrational energy spectra, or energy build-up on such a free electron?
• Rayleigh Scattering happens on the atomic level so it begs the question, "can it cause both rotational and vibrational energy level changes in atoms or molecules?."

DrClaude said:

Both Rayleigh and Thomson scattering are elastic, so they can't lead to a change in the energy levels of the scatterer.

cemtu said:

But Thomson Scattering causes electron to accelerate. What about it? How is it elastic then? Is it an acceleration in the sense of oscillation/vibration?

DrClaude said:

The charged particles doing the scattering accelerate in the sense that the direction of motion changes, but the kinetic energy stays the same. See https://en.wikipedia.org/wiki/Thomson_scattering

cemtu said:

I didn't quite understand this. Could you elaborate on it, please?

snorkack said:

Consider a light speed photon scattering from a stationary target of a fixed mass.
The energy of the photon after Compton scattering is E=E_γ/(1+2E_γ/mc²)
On the limit of E_γ<<mc², the momentum delivered on the target decreases linearly with the original photon momentum. But since the target is massive, the energy lost to the target is proportional to square of the energy of incoming photon. The redshift of the photon thus is proportional to its energy, diminishing at low energies - the limit of Thomson/Rayleigh scattering.
But the redshift never actually goes to zero if the photon energy is nonzero and target mass finite.
How does the target mass behave for Rayleigh scattering?

snorkack said:

Target mass necessarily ends up changing momentum because the momentum of the incoming proton changes. But what is the target mass for Rayleigh scattering?

cemtu said:

All electrons of an atom or molecule?

snorkack said:

And that´s the question.
One electron? All electrons? The whole atom including the nucleus? The whole molecule? A fluctuation of gas density spanning multiple molecules? Or the whole piece of solid?
Mind you, of course Rayleigh scattering from nonstationary targets must have both redshift and blueshift - according to thermal motion of the target molecules. Does the line shape of the Rayleigh scattered light match the Boltzmann distribution of the target?