Can stimulated emissions from CO2 be absorbed?

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Discussion Overview

The discussion revolves around the behavior of stimulated emissions from CO2 molecules and their interaction with photons. Participants explore the implications of coherent photon emission and absorption, particularly in the context of greenhouse gases and their effects on laser light transmission.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that when an excited molecule absorbs a photon, it emits two coherent photons, and question whether both photons can be absorbed by a second molecule.
  • Others argue that the coherent nature of the emitted photons means that if one photon is absorbed, the second should also be absorbed, though this claim is challenged.
  • One participant emphasizes that photons do not "hit" molecules like billiard balls, suggesting that absorption is more complex and can occur with incoherent light.
  • Another participant points out that while non-CO2 materials can absorb light, CO2 itself can absorb and re-emit photons, either spontaneously or through stimulated emission.
  • There is a discussion about the implications of CO2 absorption on laser beams used for measuring distances, with one participant expressing skepticism about the absorption of CO2 lasers in the atmosphere.
  • Some participants highlight the need for specificity in discussing the phenomena to facilitate meaningful discourse.

Areas of Agreement / Disagreement

Participants express differing views on the absorption and emission processes of photons in CO2 molecules, with no consensus reached on the mechanisms involved or the implications for laser light transmission.

Contextual Notes

There are unresolved questions regarding the conditions under which stimulated emission occurs and the specific interactions between photons and molecules. The discussion also touches on the limitations of current understanding regarding the absorption characteristics of CO2 in relation to laser wavelengths.

Alastair McD
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When an excited molecule absorbs a photon two coherent photons are emitted. If one of these photons is absorbed by a second molecule, since the emission was coherent then that molecule will absorb the second photon too. If the second molecule was in the ground state then it will become excited by the absorption of the first molecule. Will the second photon be absorbed too, and cause two coherent photons to be emitted, with the result that there in no net absorption?
 
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Alastair McD said:
If one of these photons is absorbed by a second molecule, since the emission was coherent then that molecule will absorb the second photon too.
Why should it?
 
Since they are coherent both are traveling together in the same direction, if one hits a molecule so will the second.
 
Photons are not billard balls. They do not "hit" molecules.

Can you stop light from a laser pointer with a sheet of paper, your hand and other things? There is your (incoherent) absorption of coherent light.
 
mfb said:
Photons are not billard balls. They do not "hit" molecules.

Can you stop light from a laser pointer with a sheet of paper, your hand and other things? There is your (incoherent) absorption of coherent light.

Paper, human hands, and other thing are not made of carbon dioxide and so do not emit stimulated radiation.
 
Alastair McD said:

When an excited molecule absorbs a photon two coherent photons are emitted.
In general, no.
If one of these photons is absorbed by a second molecule, since the emission was coherent then that molecule will absorb the second photon too.
No.

What phenomenon are you trying to open for discourse here? You have to be more specific if you hope for any meaningful replies.
 
Alastair McD said:
Paper, human hands, and other thing are not made of carbon dioxide and so do not emit stimulated radiation.
They do not emit it, but they can absorb it.
Anyway, CO2 can absorb it as well. It absorbs a photon, done. It can re-emit the photon later, either spontaneous (in a random direction) or stimulated (if the laser is still switched on). Alternatively, it can lose the energy in non-radiative ways.
 
Heinera and mfb,

Thank you for your replies. It seems that I have not explained my dilemma fully. I will try again.

When a greenhouse gas molecule absorbs a photon, the molecule becomes excited. If the molecule is hit by a second photon of the same frequency, before it has relaxed back to the ground state (by spontaneous emission), then it will emit an additional photon by stimulated emission with the "identical phase, frequency, poalisation, and direction of travel as the [incident] photon" https://en.wikipedia.org/wiki/Stimulated_emission

Perhaps I should have writtem "When an excited molecule is hit by a photon the result is two coherent photons are produced." It seems to me that it would be imposible for that coherent pair of photons to be absorbed by a molecule in the ground state since the first photon would excite the molecule and the second produce a stimulated emission.

I was hoping that I would find an expert here who could confirm my thinking, or expain to me why I am wrong citing experimental evidence. I believe that the distance of the Moon is measured using a CO2 laser, so the beams to and from the Moon cannot be significantly absorbed by the CO2 in the Earth's atmophere.
 
Alastair McD said:
When a greenhouse gas molecule absorbs a photon, the molecule becomes excited. If the molecule is hit by a second photon of the same frequency, before it has relaxed back to the ground state (by spontaneous emission), then it will emit an additional photon by stimulated emission with the "identical phase, frequency, poalisation, and direction of travel as the [incident] photon" https://en.wikipedia.org/wiki/Stimulated_emission
It can do so, it does not have to.
Alastair McD said:
I was hoping that I would find an expert here who could confirm my thinking, or expain to me why I am wrong citing experimental evidence. I believe that the distance of the Moon is measured using a CO2 laser, so the beams to and from the Moon cannot be significantly absorbed by the CO2 in the Earth's atmophere.
I don't think CO2 lasers are used, as the detection needs a good quantum efficiency and CO2 lasers emit at ~10 micrometers, too low-energetic for sensitive detectors. Wettzell uses Nd:YAG in the near infrared (~1 micrometer), not sure about the others.

The transmittance at 10 micrometers would be about 80%, see https://upload.wikimedia.org/wikipedia/commons/e/e9/Atmospheric.transmittance.IR.jpg. The presence of CO2 gives a small dip in the spectrum, so exactly at the lasing frequency transmission might be lower.
 

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