How a solid body emits a lower frequency photon than absorbed

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Main Question or Discussion Point

I’m trying to understand how a solid body changes the wavelength of radiation it re-radiates from that which it originally absorbed. I’m thinking in context to the way that the Earth absorbs higher frequency radiation from the sun, but when it re-emits the energy it’s at a much lower frequency.

What I *think I understand is that; approximating Earth and the sun as black bodies; the sun being at a much higher temperature radiates energy where the peak of the distribution is at a high frequency. The Earth absorbs the higher frequency energy and becomes “warmer”. When the Earth re-radiates the energy, because it’s at a much lower temperature, the distribution of radiation has a peak at a much lower frequency (IR).

But also I thought that heat radiation is transferred at discrete energies. Where IR radiation is absorbed in a molecule by causing rotation and vibration, and re-radiated at the same frequency. While higher frequency radiation is absorbed by promoting an electron to a high state, and when it falls back to ground state it emits a photon of the same energy/frequency. In solids I *think that electron energies exist in continuous bands, but I thought that the same would apply as in a gas, where a photon with energy hv of visible light promotes a single electron to a higher state, exactly hv greater, and then it emits another photon also with energy hv. So how is radiation ever “converted” from high frequency light to IR? I suppose that the IR is radiated due to rotation and vibration of molecules? ..... So is the question I should be asking is “how is visible light converted to kinetic energy?
 

Answers and Replies

  • #2
Drakkith
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But also I thought that heat radiation is transferred at discrete energies. Where IR radiation is absorbed in a molecule by causing rotation and vibration, and re-radiated at the same frequency.
No, not necessarily. Once a molecule has absorbed energy and started vibrating, rotating, etc, it can often get rid of that energy in a continuous manner instead of a discrete manner. For example a molecule can collide with another molecule and lose half of the energy that it absorbed. Or a third. Or some other fraction. Or it could gain even more energy.

While higher frequency radiation is absorbed by promoting an electron to a high state, and when it falls back to ground state it emits a photon of the same energy/frequency.
This is only an accurate description for single atoms. Molecules have more degrees of freedom in which to interact and, and the larger the molecule the more degrees of freedom it has. This means that an electron can lose energy in more ways than just emitting a photon.

So is the question I should be asking is “how is visible light converted to kinetic energy?
Imagine that a photon is absorbed and the energy is transferred to the molecule in the form of the rotation of part of the molecule around another part. A collision between this molecule and another could transfer that rotational energy into kinetic energy for the 2nd molecule, sort of like a bat hitting a ball.
 
  • #3
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Thanks for the explanation!

If i can follow up though, I'm trying to think more specifically about higher frequency radiation (visible light). In solids, as opposed to gasses is there more possibility that a molecule can absorb photons through vibration, rotational modes, instead of electron promotion?

If a solid does absorb a photon through electron promotion, can the energy, or part of it be converted to rotation or vibration?

Also, just so I'm clear, I thought molecular vibration and rotation were "kinetic energy", is that correct?
 
  • #4
Drakkith
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If i can follow up though, I'm trying to think more specifically about higher frequency radiation (visible light). In solids, as opposed to gasses is there more possibility that a molecule can absorb photons through vibration, rotational modes, instead of electron promotion?
I believe so, but I confess I'm not an expert, so I can't offer any details.

If a solid does absorb a photon through electron promotion, can the energy, or part of it be converted to rotation or vibration?
That's a good question and I wish I knew enough to answer it. I think it can, but I don't know what the requirements would be for that to happen.

Also, just so I'm clear, I thought molecular vibration and rotation were "kinetic energy", is that correct?
I'm not sure. I was just thinking of translational motion as kinetic energy.
 
  • #5
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Awesome. I think you've cleared things up enough so that I can refine my question and possible post in the "Condensed matter" forum. It seems like this would be right up there alley. Also seems like a course in statistical mechanics would help. If I ever have time someday, I think I'll take one.
 

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