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Can stimulated emissions from CO2 be absorbed?

  1. Sep 6, 2015 #1

    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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  3. Sep 6, 2015 #2

    mfb

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    Why should it?
     
  4. Sep 6, 2015 #3
    Since they are coherent both are travelling together in the same direction, if one hits a molecule so will the second.
     
  5. Sep 6, 2015 #4

    mfb

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    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.
     
  6. Sep 6, 2015 #5
    Paper, human hands, and other thing are not made of carbon dioxide and so do not emit stimulated radiation.
     
  7. Sep 6, 2015 #6
    In general, no.
    No.

    What phenomenon are you trying to open for discourse here? You have to be more specific if you hope for any meaningful replies.
     
  8. Sep 6, 2015 #7

    mfb

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    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.
     
  9. Sep 7, 2015 #8
    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.
     
  10. Sep 7, 2015 #9

    mfb

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    It can do so, it does not have to.
    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 this plot. 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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