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Photon absorption

  1. Jun 2, 2009 #1
    I've been wondering, if a photon is very far off any atomic resonances, can it still be absorbed by the atom? or will there be compton scattering? or will the photon pass through the atom? or is there something else I'm not considering...

  2. jcsd
  3. Jun 3, 2009 #2
    For absorption to occur, photon energy must be equal to the difference of two energy levels of the atom. In any other case, it depends on the scattering cross-section of the particular atom.
  4. Jun 3, 2009 #3


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    Above a certain energy, the photon can ionize the atom.
    There can always be Compton scattering.
    At any energy, most photons just pass through.
  5. Jun 3, 2009 #4
    Hi there,

    If you are very far away from the atom, the photon has very little chance of interacting with the atom. Eventhough not impossible, the photon has most chance of passing right by.

    Another possibility that you have not mentioned is pair-production. If the photon has enough energy, it can create a pair of electron-positron.

  6. Jun 3, 2009 #5
    If the photon energy is above the absorption threshold, it can be absorbed - the corresponding cross section is different from zero. But amongst other different possibilities the absorption may be much less probable than the other events.

  7. Jun 3, 2009 #6
    There are several ways a photon can interact with an electron; photoelectric effect (bound electrons), Compton scattering (off both free and bound electrons), pair production (weak compared to nuclear pair production). The photon can interact directly with the nucleon (nucleus) in many ways including pair production, photoproduction (e.g., gamma + neutron --> pi minus plus proton), photonuclear (e.g., gamma,n reactions, etc.). If one calculates the geometrical cross section of a nucleus, it will be many times larger than photonuclear cross sections, so the nucleus is "transparent."
  8. Jun 4, 2009 #7
    Thanks for the replies.

    I was actually thinking of a very low energy photon interacting with an atom, so there's probably no pair production or nuclear interactions...

    I was also thinking that since the photon is of low energy, there shouldn't be any atomic excitation. However, I was looking thru the optical bloch equations for a 2-level atom (2 internal energy levels) and the equations show that there always is a non-zero population on the higher level. I can't seem to figure out the excitation mechanism. And is there a way of calculating the probability of photon absorption?

    Thanks again.
  9. Jun 4, 2009 #8
    If the photon energy is smaller than the energy difference in your two-level system (ћω<∆E), it cannot be absorbed. Your system, once prepared in the ground state, should stay unexcited forever. But there is also a multi-photon absorption mechanism. If your incoming wave is not one photon but an intense coherent wave with many photons in it, there may be multi-photon absorption: the energy conservation law is now different, like 2ћω=∆E or generally nћω=∆E. This is most probable reason in your problem.

    Last edited: Jun 4, 2009
  10. Jun 5, 2009 #9
    Even when the photon energy is not near atomic transitions, the atom can scatter incident radiation. Rayleigh scattering (of sunlight in air) is an example.
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