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Classical explanation of Photodissociation?

  1. Oct 12, 2011 #1
    Can someone explain to me how an atom can absorb the energy of an incident photon without being ionised? What is the photon energy transformed into and how is it transferred between the photon and the atom (what part of the atom absorbs the energy?). I can't find anything helpful online - the resources I've managed to find are either too simplistic or far too complex. I'm hoping that this can be explained without a quantum mechanical treatment? I appreciate any help.
  2. jcsd
  3. Oct 12, 2011 #2


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    Let's take the hydrogen atom for the sake of simplicity. Say the atom isn't excited and thus the electron is on the first orbit. The electron needs about 13.6 eV to leave the atom. So if you "fire" a photon with less energy on the atom and the electron absorb it, the electron will just move to a further orbit (then the atom is excited and I think the electron will move to the first orbit again, emiting another photon in the process of changing of orbit). The atom in this case isn't ionized. However if you "fire" a photon with more than 13.6 eV and that the electron absorbs it, it will leave the atom and thus the atom will be ionized.

    Since photons carry momentum, the atom, after absorbption of the photon will be left with a different momentum than before the absorbption.

    I am not sure what would happen if a nucleous would absorb a photon though. My guess is that it will transfer momentum to the atom and nothing else special would occur, but I'll let experimented people talk. :)
  4. Oct 12, 2011 #3
    An atom or a molecule can absorb energy, from a photon or from an electron or another atom or molecule, and be left in an excited state. Depending on how much energy is absorbed, that excited state could be a higher-energy bound state (electronic or vibrational or rotational), an ionized atom, or a disassociated molecule.

    You can think of it as you absorbing energy from, say, the springs of a trampoline, and using it to rise up in the air; energy hasn't been created or destroyed, it's all still there but in a different form. Now you can fall back and transfer your gravitational potential energy back into spring energy, or into biological disassociative energy and heat if you splatter on the ground.

    It doesn't make sense to talk about "how" the energy is transferred in a photon absorption, or what part of an atom absorbs energy - given an amount of absorbed energy, the result can be a variety of different excited states of the atom or molecule as a whole.
  5. Oct 12, 2011 #4
    Once you say "photon" you've left the classical world, so it will be difficult to find a classical explanation. About the best you can say classically is that there are, for unknown reasons, stable configurations of electrons and protons and that the electrons can be shaken loose with electromagnetic waves. How much energy is required from the incident wave is unknown (because how the electrons maintain a stable state is also unknown). To get to a "photon" of energy classically requires unrealistically strong em waves.
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