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What happens after an electron is excited by a photon?

  1. Jul 4, 2012 #1
    I understand the concept of an electron being raised from it's ground state by a photon with the correct wavelength, but then what? I ask because if the electron remained in it's excited state then surely shining a bright light on a material for long enough would 'saturate' it's electrons and cause it to become transparent to a given wavelength. But then if the electrons fall from their excited state then surely the energy of the photon, need to be conserved, is re-emitted by the material?

    Also if electrons can only absorb wavelengths of light at exact frequencies to promote them then why are the majority of materials opaque to so many frequencies of the visible spectrum?
     
  2. jcsd
  3. Jul 4, 2012 #2

    mfb

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    This is one option. The other options would be to produce some heat, to excite other electrons and probably something I forgot.

    They always have a natural width of the transition and a doppler broadening due to their temperature. However, this is not relevant here. If the electron can reach a free band (in metals or semiconductors), you get a full range of possible energies. In other materials, you can have other allowed transitions between energy levels in the material.
     
  4. Jul 5, 2012 #3
    What is the mechanism by which the electrons heat up?

    Does this mean that transparencies would change at much lower temperatures, e.g. the ground state of electrons at 0K?

    Wouldn't this cause absorptions at specific frequencies such as that exhibited by interstellar gases, I still don't understand how one material can absorb such a wide band of photon wavelengths
     
  5. Jul 5, 2012 #4

    mfb

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    Not the electrons, the material can heat up, which corresponds to atoms and sometimes electrons moving around.

    In some situations, the absorption coefficient for a specific wavelength can depend on the temperature, right.

    Let's take a metal, as it is quite easy to understand there:
    You have some electrons which are bound to specific atoms, they are not relevant here.
    In addition, you have some electrons which are not bound to specific atoms. They are similar to free charges in a very large box, and therefore the allowed energy levels are extremely dense. For every photon in a wide energy range (especially all photons of visible light), every charge can absorb it and go to a higher energy level.
     
  6. Jul 5, 2012 #5
    In the case of a metal if the electrons are delocalized then how can they be promoted?
     
  7. Jul 5, 2012 #6

    Drakkith

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    Delocalized just means that the orbital of the electron isn't around just one atom, but around multiple. See here: http://en.wikipedia.org/wiki/Delocalized_electron
     
  8. Jul 11, 2012 #7
    Ok then, how does the electron cause a heating of the atom through it's excitation? And does this mean that much of light energy falling on a surface is turned into heat or is it just a nominal amount. And can any wavelength cause a heating effect - e.g. a focused gamma ray burst will cause extreme heating?
     
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