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Why are photons released when electron jump states?

  1. Nov 20, 2013 #1
    I've always taken this for granted. Now I am looking for an answer.

    When electron jumps from a higher orbit to lower orbit it releases energy. Why is the energy in the form of photon?

    I will take another example which will make my question easy to understand. When two electrons are kept close, they have high potential energy as they repel each other. Since force causes movement, energy will be in the form of kinetic energy.

    Similarly when electron jumps states, why is the energy in the form of light or photon? Does my question make sense?

    Thank You!
  2. jcsd
  3. Nov 20, 2013 #2


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    You only have half of the picture. You might want to look up what is known as the dipole transition.

    When an atom (and it is the whole atom, not just an electron) decay from a higher energy state to a lower energy state, the change is not just a change in "n", the principle quantum number, but there must also be a change in "l", the orbital angular momentum quantum number. It means that an electron simply can't go from, say, the 3s state to the 2s state (no change in angular momentum quantum number). It can go from a 3p state to the 2s state. This is what is meant by the selection rule in such a transition. Only certain transitions are allowed.

    Thus, in such a transition, due to conservation laws, the "energy" being created must also carry the same change in angular momentum. And naturally, photons have a spin angular momentum of 1.

  4. Nov 20, 2013 #3


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    ZapperZ is absolutely correct, but i think you need something less technical.
    I'll try to answer without any mathematics, only qualitatively, just for you to get a conceptual/qualitative understanding.

    Your understanding (in bold) is correct. When the electrons are kept close, the potential energy is indeed very high. But in order to answer your question, you need to understand what the "potential energy" is. When you have two charged particles interacting, the potential energy includes not 2.. but.. 3 systems!Except from the electron and the nucleous, it also includes the electromagnetic field (photons) via which the electron & nucleous interact. Now, when the electron jumps from an excited state to a less excited state, the energy of the atom is reduced. Perhaps you could ask, "Why does it reduce? It could go to another configuration keeping the same energy, as you can have trajectories with different radious with the same energy", and i think this is part of your question when you're saying that kinetic energy should increase. Well, quantum mechanics tells you that you cannot have that if you want your system to be stable, in order for the atom to be stable it has to reduce its energy, so the "increase of kinetic energy" (in a classical sense) cannot happen. In order to reduce its energy, the atom needs another system to give its energy to. If there is no other such external system, then the electron will NOT jump. In our case, this other system is the electromagnetic field, which is included in the potential energy as we said before, hence the energy given to the E/M field has the form of a photon.

    A small but important note:

    Don't consider the photon to have come out of "nowhere". The electromagnetic field is ever-present in all space even if you don't have photons around, and this is key to understand. When there are no photons around, we say that the E/M field in its vacuum state. When you have an electric charge (e.g. electron and nucleous), this charge interacts with this ever-present vacuum state of the E/M field. If the charge give energy to the field, this may give rise to photons.
    Hence, the photon is just a different manifestation of what is already there.
    Last edited: Nov 20, 2013
  5. Nov 20, 2013 #4
    That explanation strikes me as being backwards. It states that the energy must be carried away by a photon because the selection rule requires a particle with spin 1. But the selection rule itself is a consequence of the fact that photons have a spin 1 to begin with so that is circular logic.
  6. Nov 20, 2013 #5


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    Not always the case. There is also a process called collisional deexcitation, in which an excited atom or molecule is deexcited by collision with another. This takes place without photon emission, and the energy released goes instead into kinetic energy.
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