Scattering of light by air molecules -- Is it just for our atmosphere or for all gases?

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Doctor U
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As far as I know, the sky is blue because of the natural vibration of the molecules of the air enter in a ressonance with the ultraviolet spectrum, so when the sun light hits the air molecules the scattering occours more on the ultraviolet part, but blue part of the visible spectrum is more scattered and that's why we see a blue sky, or a red sky near the sunset. I'm especulating if this is a characteristic of our atmosphere or is a commun natural vibration for all the gasses we know? The possibility of a green sky where the natural vibration of the air molecules is different from the one we know is possible or it's commun for all the gases we know?
 

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  • #2
snorkack
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Not all gases we know.
Rayleigh scattering where the scattering is strongly increased in the shorter wavelength. It is proportional to fourth power of frequency for a constant refractive index - but refractive index always increases with frequency as well except at wavelengths that have strong absorption.
The obvious possibility of green sky is to have a gas which absorbs in blue but not green. Green would still bew more scattered than red.
 
  • #3
sophiecentaur
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Have you done any background reading about light scattering? This wiki article is a good starter and could give you a better idea of how to phrase your questions to PF and many of the answers lie within that article.
Resonance is not involved with most scattering events and the particles can be treated just as particles. Rayleigh scattering is the best known and gives a cause for 'why the sky is blue' etc..

Edit: @snorkack is right here: Absorption is a quantum effect and optical interactions are more associated with the energy levels in atoms, rather than molecular energy levels.
 
  • #4
DrClaude
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optical interactions are more associated with the energy levels in atoms, rather than molecular energy levels
This doesn't make sense to me. Do you mean electronic energy levels compared to ro-vibrational energy levels?
 
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sophiecentaur
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Yes
 
  • #6
sophiecentaur
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Isn’t it true to say that interactions with molecules involve lower energy levels [changes]? I’m not implying that molecular Hydrogen doesn’t have changes in the levels of the electrons in each atom.
 
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  • #7
snorkack
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There are actually a lot of molecular orbital excitations that are visible and which atoms plainly do not have. Like excitations to antibonding molecular orbitals. Which atoms do not have. And halogens spectacularly do:
http://wwwchem.uwimona.edu.jm/courses/CHEM3101/Halogens2.html
For the "pale yellow" F2 see
https://pubs.acs.org/doi/pdf/10.1021/ja01586a007
even nicer if you can see past the first page.
Fluorine has the absorption maximum around 285 nm.
I see that the units are in l/mole/cm. Since gas in room temperature has about 24 l/mole, I interpret it to mean that at 285 nm, optical depth of 1 (63 % light absorbed) is reached by 4 cm fluorine.
In visible light, at 400 nm, 80 cm fluorine would have unit optical depth, and at 450 nm, 4 m is needed. So fluorine is colourless in small vessels or small puffs, and you need long (lab scale) cells to perceive the pale yellow.

With their 10 cm cell, the absorption at 500 nm was immeasurable.
If you could somehow see a 10 km layer of 1 bar molecular fluorine, the distance where Rayleigh scattering matters, would it look green (because green passes and is more scattered than red) or would the absorption tail at 500...550 nm be appreciable enough to block green as well and make fluorine red/brown?
 
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  • #8
sophiecentaur
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There are actually a lot of molecular orbital excitations that are visible and which atoms plainly do not have.
Thanks for that information. I was obviously being too sloppy with my statement.
 

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