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How can a atom generate photons ?

  1. Jan 5, 2009 #1
    I don´t understand how can an excited atoms generate photons, ex. a incandescent lamp.
    Does vibrating eletrons emit eletromagnetic fields ?

    Thanks a lot
     
  2. jcsd
  3. Jan 5, 2009 #2
  4. Jan 5, 2009 #3

    ZapperZ

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    Light from incandescent light bulb is NOT from "atoms", but rather from the solid itself that makes up the filament of the light bulb. The filament is made of a metal, usually tungsten. When heated, the ions that make up the crystal lattice of the tungsten vibrate. The ions can be taught of as a chain of "+" and "-"... thus, you have oscillating dipoles that increase in amplitude as the temperature increase. That's why it gets brighter when it gets hotter.

    You can tell that light coming from such light source is different than those coming from, say, a discharge tube that uses pure gases. For one thing, you will see NO DISCRETE LINES when you look at the light from an incandescent light bulb, unlike the discharge tube. Try it using a spectrometer. So this already tells you that you're not seeing an atomic spectra.

    When atoms form a solid, they lose a lot of their individual identity (read our FAQ on photon transmission through a solid). If you don't learn anything else from this post, that last point is something you should remember.

    Zz.
     
    Last edited: Jan 5, 2009
  5. Jan 5, 2009 #4
    But is not a photon emited when a electron of this ion change energect level ( in orbit ) ?
    I did imaginate that molecules from material collides ( because temperature ) each other, making electrons change betwen energy levels.
     
  6. Jan 5, 2009 #5

    ZapperZ

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    Er... read what I had already written. There's no "atomic transition" at all here.

    There are MANY ways to generate EM radiation. The radio antennas that generate the radio waves used no atomic transition. It is just the current oscillating back and forth along the antenna. I can take a charge and put it at the end of a spring and let it oscillate. That creates EM radiation as well. In none of these was there any "atom" or atomic transition involved. This is what I meant as "oscillating dipoles" in my previous post.

    When atoms form a solid, they lose a lot of their individual identity. <-- You need to fully understand the significance of what I said here.

    Zz.
     
  7. Jan 5, 2009 #6
    um.. is another way to produce EM radiation, or in essence photons, through the virtual photon exchange among local, oscillating cahrged particles (the virtual photons that become reality as they "escape")?? Is this how "oscilating dipoles" generate photons as they are undergoing enough thermal (as caused by the heat in the light bulbs) and kinetic energy to account for such vibrations. Thus, wouldn't these influxes of position seem to well wnough create phton release, or is that wrong...
     
    Last edited: Jan 5, 2009
  8. Jan 6, 2009 #7
    Eh, is the solid not made of atoms? Metals are made up of cations (atoms which have lost electrons) in a "bath" of electrons.
    Light from a tungsten bulb is very similary to black-body radiation. It has a spectrum that is related to the temperature. The only difference between this light and the one from the discharge tube is in the number of vibrational modes (or energy levels) of the atoms present in the material. In a discharge tube the electrons are bound so the energy levels are limitted, unlike in a metal in which the electrons are free. The presence of a few descrete lines in one case and a million descrete lines in another (what is commonly called a continuous spectrum) does not change the fact that atoms and electrons are the sources of the radiation. What else is there to radiate anyway?

    The FAQ to which you referred is questionable on the aspect of atoms losing their identity.
     
    Last edited: Jan 6, 2009
  9. Jan 6, 2009 #8

    ZapperZ

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    Then "Solid State Physics" would be redundant. It could easily be stuffed into "Atomic and molecular physics".

    The conduction band in a metal cannot occur with a few atoms. It requires a collective property of ALL the atoms to form the conduction band. A copper atom does NOT have a conduction band. A copper SOLID does. The band gap in a semiconductor isn't a property of those individual atoms that make up the semiconductor.

    The "vibrational modes" in a solid, which are the phonon modes, is exactly the reason why the solid is different than the individual atom. These phonon modes are completely absent in individual atoms.

    You need to read up on what physicists such as Robert Laughlin and Phil Anderson categorize as "emergent phenomena". As Phil Anderson has said, "More Is Different".

    Zz.
     
  10. Jan 7, 2009 #9
    It is one thing to say atoms in a solid behave differently from isolated atoms atoms in a gas. Of COURSE! That's why they call one a gas and the other a solid. It does not mean atoms in a solid have lost their identity.

    Of course, with a few atoms, you have less orbital splitting and thus fewer energy levels. It is called a band because it has a large number of energy levels with small gaps between them so it is easy for an electron to jump from one to another.
    That is beside the point. You are pointing out differences between free atoms and atoms in a solid. My point is simply that you have ATOMS in both cases. If you ask WHAT is vibrating in the phonon, it still boils down to ATOMS. They fact that the frequences and phases are correlated does not change this fact!
     
  11. Jan 7, 2009 #10

    ZapperZ

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    Read what I wrote:

    If you have ever performed any photoemission experiment, the way I have done, you will see that within the first few eV of the band of the solid, the spectra is NOTHING like what you would get out of the individual atoms. It is only when you do CORE level photoemission that you start to get some resemblance of the spectra of the individual atoms. However, most of the behavior and properties of the solid comes predominantly from the first few eV of the band structure, i.e. the valence band. My avatar comes from the photoemission spectra of the first few eV of a cuprate superconductor, which looks NOTHING like the spectra of copper or oxygen. Yet, this band structure is what governs the superconducting behavior of the material, which you do NOT get out of the individual atoms. It dictates if the material is a metal, a semiconductor, an insulator, etc. In more exotic material, the long-range collective behavior dictates if it is a paramagnet, ferromagnet, antiferromagnet, etc. Nowhere are any of these the behavior of individual atoms. The valences orbital of these atoms look NOTHING, not even close, to the valence band of the solid.

    The band structure of a solid is not created out of one, two, three, or even 200 atoms. It is created out of the collective behavior of ALL the atoms. That is the only way one can form the Fermi surface of a metal, which does not occur and makes no sense for individual atoms.

    Zz.
     
  12. Jan 7, 2009 #11
    Look, I also do photoemission experiments on a daily basis. Again you are not reading what I write. Nobody is refuting the fact that solids behave differently from single atoms. The statement I AM objecting to is the following:

    Which gives the impression that properties exhibited by solids do not originate from the constituent atoms (whether it be by collective action or not). There is a reason why metallic copper and metallic aluminum have different properties and it can be traced to the constituent atoms! This is my last post on this issue.
     
  13. Jan 7, 2009 #12

    ZapperZ

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    But they do not! Just look at the spectra and that's that! The atom themselves need not even be there, but simply replaced by a series of positive and negative charges with the correct mass! The individual properties of EACH atoms are gone and has no relevance in the spectra generated by the tungsten filament! It is the collective behavior of ALL the constituents, NOT of ONE of them, that causes the continuous band!. I bet the OP didn't know of such differences in the first place!

    And I could easily point out to the fact that using the SAME identical atoms, but arranged in different ways, that I could get completely different solid. Example: graphite and diamond. Without knowing that they are made of carbon atoms, no one would guess that they are made of the same atoms. Arranged differently in different crystal lattice, you get completely different behavior. How does the SAME atoms produce such different outcomes? Easy! Because the individual behavior of each atom no longer plays a significant role in the collective property of the solid. In this case, a different arrangement of the atoms creates something completely different, not only from each other, but also from the property of the individual atom.

    Zz.
     
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