Thomson - Rayleigh Scattering: Rotational & Vibrational Energy Levels

[cemtu]

TL;DR Summary

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?."

We know from molecular spectroscopy that incoming light on a molecule can change a molecule's rotational, vibrational and electronic energy levels.

If the incoming light is,

• 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)

We also know that,

• 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.

Taking into account that both Rayleigh and Thomson most probably happen at low energies and vibrational and rotational energy changes of molecules happen at microwave and infrared spectra:

QUESTION:
It is clear that,

• 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]

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

 

[cemtu]

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

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]

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?

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]

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

Oh I get it, you are talking only about Thomson and Rayleigh cuz they are elastic scattering, I get it now, I got confused for a moment thinking about Compton...

 

[snorkack]

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

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?

 

[cemtu]

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?

Target mass oscillates as the elastic scattering happens due to the wave nature of light, which happens because the electrons interact with the Electrical Field and this field causes the electrons to accelerate -> vibrate -> Change in direction(???) only not the value of velocity itself.

I don't even know how to vibrate or oscillate without changing the absolute value of velocity. How can you both oscillate and only change the direction of motion at the same time but not the absolute value of it?

 

[snorkack]

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]

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

All electrons of an atom or molecule?

 

[snorkack]

All electrons of an atom or molecule?

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?

 

[cemtu]

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?

I dont know... :(

 

[snorkack]

A simple order of magnitude estimate:
At 300 K, free electrons should have average thermal velocities of around 60 km/s. Which is about 1/5000 the speed of light. Thus Thomson scattered light should have broadening in the order of magnitude of 1/5000 (or maybe twice as much?).
Whereas the redshift of Compton scattered radiation is in the magnitude of 2E_γ/mc²,
so the Compton scattering redshift should match thermal broadening around 50 eV, or 25 nm. Vacuum UV. Strongly absorbed, nuisance to work with. Is this the reason Thomson/Rayleigh and Compton scattering are seen as different things? That the transition region is hard to explore?

 

[nightvidcole]

If visible light scattering off a molecule causes a change in vibrational or rotational energy, it is termed Raman scattering rather than Rayleigh or Thomson scattering.