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Evidence of single photon absorption?

  1. Feb 3, 2013 #1
    Hey all,
    Does anyone know of a paper or experiment that verified that a single photon is absorbed in exchange for electron excitation?

    Thanks
     
    Last edited: Feb 3, 2013
  2. jcsd
  3. Feb 3, 2013 #2

    ZapperZ

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    Not sure i understand the problem here. The typical photoelectric effect is a prime example of single photons being absorbed to excite electrons into the vacuum state. If one accounts for all the energy involved on the photoelectron side, one gets the energy of a single photon. This is not evidence enough?

    Zz.
     
  4. Feb 4, 2013 #3

    Jano L.

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    This is quite a difficult issue. I do not think that today photoelectric effect is taken as sufficient confirmation that energy is exchanged discontinuously by light quanta. There are semi-classical theories of it as well (by Lamb and others), where light is modeled by continuous EM waves and matter described by the Schroedinger equation.

    According to the book

    Knight, Allen: Concepts of Quantum Optics,

    only the later experiments in 70's and later were able to show the presence of quanta in coincidence experiments. The book is very nice and contains also the original papers on the subject.

    However, there are scientists who do not believe in the necessity of quanta even for such coincidence experiments. In my opinion, the issue is still a bit controversial.
     
  5. Feb 4, 2013 #4

    ZapperZ

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    Are you aware of multiphoton photoemission phenomenon?

    Do you have a semi classical description for that?

    Zz.
     
  6. Feb 4, 2013 #5

    Jano L.

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    I am surprised. Could you explain shortly what aspect of that phenomenon requires multiple quanta? Or can you suggest some reference to learn about that?
     
  7. Feb 4, 2013 #6

    ZapperZ

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  8. Feb 4, 2013 #7

    Jano L.

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    Thank you. I will look into that.
     
  9. Feb 4, 2013 #8

    Cthugha

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    Really? Are any of these not crackpots? I am not aware of any serious scientist still claiming that antibunching (and its variants) can be explained by theories in which the light field is not quantized.
     
  10. Feb 4, 2013 #9

    Jano L.

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    Not all of them I think. How do you recognize a crackpot?
     
  11. Feb 4, 2013 #10

    ZapperZ

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    There are still a minority of physicists who are clutching on to SED and other semi-classical model of light, but they typically only use the naive photoelectric effect results to show that they can get the same outcome.

    Unfortunately, these theorists (and yes, the ones that I'm aware of are theorists - haven't come across an experimentalist that adopt this idea) often either ignore, or are unaware, that we have gone beyond that simplistic photoelectric effect. As I've said, the general photoemission phenomenon is considerably richer than the photoelectric effect. Multiphoton photoemision, resonant photoemission, angle-resolved photoemission, etc.. etc. have not only severely tested the photon model, but also are so clearly-known, we use these technique to study things with. And I have seen ZERO semi-classical description attempting to account for those photoemission phenomena. Zilch.

    Zz.
     
  12. Feb 4, 2013 #11

    Cthugha

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    Claiming to have a great model explaining everything, while simultaneously only showing a vague validity of that model for a very specialized situation is a good start. Apart from that the crackpot index helps:
    http://math.ucr.edu/home/baez/crackpot.html.

    Note that I am not talking about people like Lamb who simply think that the term "photon" is misleading and often misunderstood.

    Exactly that was my impression, too.
     
  13. Feb 4, 2013 #12
    Zapper - the reason I'm questioning whether one photon can in fact excite one electron is because the excitation time for an electron elevating is on the order of nanoseconds. During that time a photon can cycle as much as 10^7 cycles! So that begs the question what really is going on here? If the photon is discretized why is the excitation jump not instantaneous itself? There seems to be some absorption time at play.

    IxR
     
  14. Feb 4, 2013 #13

    ZapperZ

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    10^7 cycles?

    You do know that in the typical photoelectric effect, the electron is not bounded to any particular atom, and that it is essentially a free electron in the conduction band?

    Secondly, I can show you experiments where the photoemission response time is in the order of picoseconds and femtoseconds! I did a literature survey of photocathodes a while back:

    http://physicsandphysicists.blogspot.com/2007/05/cathodes-for-photoinjectors.html

    Read my blog in my signature on "See an electron lately?" on the response time of GaAs photocathode. All of these timescales are way faster than you think!

    Zz.
     
    Last edited: Feb 4, 2013
  15. Feb 4, 2013 #14
    Haha I agree on that note and I'll get to your blog in a moment but let me ask you this: are electrons in the conduction band more easily liberated than electrons bound to atoms?

    IxR
     
  16. Feb 4, 2013 #15

    Cthugha

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    Just a question: Where did you get this number? Could you give a reference? I am not aware of any experiment where the EXCITATION time takes that long. It may be that the time until the electrons escape from the material takes that long if the electrons need to go a rather long way to the surface via ballistic transport, but I never heard about excitation taking that long.
     
  17. Feb 4, 2013 #16
    Your question is in fact a good one, a quick scholar search turns up femto and subfemto time scales. Until I find a nanosecond time scale I stand corrected.
     
  18. Feb 4, 2013 #17

    ZapperZ

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    The typical workfunction for metals ranges from 3 to 6 eV. Compare that to the ionization energy of the valence electron in your favorite atom.

    Zz.
     
    Last edited: Feb 4, 2013
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