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How does an electron absorb energy from a photon

  1. Jan 23, 2010 #1
    and what does it mean by absorbing?

    I understand when electron are subjected to higher potential they move towards the higher potential and therefore their kinetic energy increases.
  2. jcsd
  3. Jan 23, 2010 #2


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    Your question is pretty broad and there's a lot of contextual conditions as to how/when the electron absorbs a photon.

    Starting in a classical setting what you say about potentials is correct. If you consider an electron in a simple fixed frequency passing EM wave then it will be accelerated cyclically by the wave (like a cork bobbing in the ocean) but it will also as this happens radiate its own EM wave thus scattering a small part of the passing wave.

    Now consider the same situation but with the wave only turned on for a finite period. Chances are the electron will not be exactly at rest (in the frame where it was initially at rest) at the instant the wave get's turned off and thus will have picked up a bit of momentum and thus kinetic energy (KE).

    Now you can get more complicated in the shape of the wave, not a single frequency but rather a pulse. The electron may then "surf" this pulse and pick up quite a bit of KE. (This is what happens in those particle accelerators...controlled electron surfing [insert "wipeout" theme music here]) You can also consider cases where the electron is orbiting the nucleus of an atom in which case the passing wave effects both nucleus and electron. The interaction is more complicated but the re-emitted waves from positive nucleus and negative electron will tend to cancel out. If the wave is in sych with the orbital period of the electron it can (classically) spiral into a higher orbit... or the reverse as well if conditions are right.

    Now once you use the term "photon" you are invoking quantum mechanics so we have to consider quantum theory. Without a long winded exposition on that just suffice it to say in QM all interactions become quantized and probabilistic. The example of the passing fixed frequency EM wave is then resolved into a number of passing fixed frequency (or equiv. fixed momentum) photons. We describe the electron's scattering of the wave as its absorption of incoming photons and re-emitting photons in different directions. Or you can equivalently describe the photons as bouncing off the electron. Its just a matter of taste. Fundamentally the photons aren't wearing name tags to let us say "that's the same one which came in" or "that's a different photon all together". (In fact even pretending such distinctions can be made will lead to incorrect predictions...but that's another story.)

    But as you resolve the action of single photons (turn down the power on the wave very very low) then you'll see that probabilistic discrete behavior. The electron will seem to suddenly jump in energy and momentum as a photon bounces off it. Note too that since the electron changes energy, the scattered photon's energy will be different than the incoming one's.

    We can then say the electron absorbed some of the photon's energy or vis versa.

    Note that someone moving in the direction and at the same speed as the electron after the scattering event would see a moving electron stopped by being hit with the photon so as far as they're concerned the photon absorbed the electron's energy. Someone else in the frame where the electron is initially at rest will see the electron picking up energy from the photon. It's all relative.

    Note also that for a free electron floating in space you'll only see a scattering type interaction, i.e. a photon hits the electron and a photon leaves. If however the electron is bound in say an atom then the photon can be wholly absorbed kicking the electron up into a higher energy orbit (now discrete orbits since we've invoked QM). Similarly the electron can drop back down into a lower orbit emitting a photon.

    Now if you want to resolve the interaction further ... well you can't. By the uncertainty principle we can't resolve the electron-photon interaction into parts or component fields beyond the statements such as "the electron absorbed/emitted a photon". The very idea of "a photon" comes from this limit on our ability to resolve EM interactions. What we can do is quantify the probabilities for various cases of interactions and add them up to resolve gross behavior to very high precision...and we note in the aggregate we recover the "classical EM fields pushing charged point particles around" behavior.

    There is my best layman's answer to your question. I hope it was helpful.
  4. Jan 24, 2010 #3
    Thanks 4 answering, the scattering part was out of my head. What i would like to know was when an electron is orbiting inside an atom.

    How an photon is absorbed by an electron and acquire higher energy. Is it just that the electron is being hit by the photon and strike out of its orbit???
  5. Jan 24, 2010 #4


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    Basically. It might be better to say the photon is striking the atom. But these aren't BB's or bowling balls. We can't really resolve the intimate details, can't really even allow that there exist details beyond the statement that the atom absorbed the photon.
  6. Jan 24, 2010 #5
    when a photon hit an atom...the electron in shell may go to higher shell, i.e. get higher energy (this happens only when the incident photons having more energy than the binding energy of electron!!!)..while doing so a x-ray (fluorescence) is emitted, which is x-ray photon.
    hope this helps
    Ps: photon interaction with an atom or electrons results in many emission..
    for e.g., Compton scattering, fluorescence, Auger electrons, photo electrons, shake-off electrons, etc
  7. Jan 24, 2010 #6
    I see this kind of statement all the time and wonder whether I should try to correct it or just let it go. I have to accept that it is POSSIBLE to do physics in this way...to analyze an interaction without bothering to ask how the internal details play out. But to say that it's impossible to resolve the details...well, this seems very wrong to me. The Schroedinger picture gives a very straighforward explanation of atomic absorption based on the interference of two orbital wave functions creating a tiny oscillating dipole. The energy exchange process is then exactly the same as the energy absorption of any classical antenna in an electromagnetic field. That is one of the great virtues of the Schroedinger picture and there is nothing really mysterious about it.
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