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Wavelengths for ionization

  1. Nov 25, 2009 #1
    If a photon is used to ionize an atom then what happens to the photon? If the wavelength of the photon corresponds to the ionization energy, then the photon is absorbed/annihilated, yes? What if the photon has more energy. Is a lower energy photon emitted during ionization?
  2. jcsd
  3. Nov 25, 2009 #2


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    Yep, the photon would be absorbed (annihilated). The energy that was in the photon would go into releasing the electron, and if there were any energy left over, it becomes kinetic energy of the electron. So the more energetic the photon, the more energetic (faster-moving) the electron that comes out.
  4. Nov 25, 2009 #3
    If the photoemission is via the photoelectric effect, there is no secondary electron. The photon energy is 100% absorbed. If there is ionization, there are usually atomic emission x-rays etc. If the photon interaction is via Thomson scattering, there is a secondary electron due to photon scattering..
    Bob S
  5. Nov 30, 2009 #4
    Thank you for the answers. However, there was one other point I was getting to. If the photon has an energy such that, if absorbed by the atom, it falls "between" two quantum states, then this photon cannot be absorbed, correct? Since the extra energy can't disappear, the absorption cross section is zero?
  6. Nov 30, 2009 #5
    Photons can scatter inelasitically off of bound electrons in the atom. This is Compton scattering, usually leading to an ejected electron, but occasionally the electron could end up in a higher bound quantum state. One example might be a photon with an energy exceeding 0.75 x 13.6 eV inelastically scattering off of the bound electron in a hydrogen atom, leaving the electron in a higher p state. Because two photons are involved (one incident, one leaving), the cross section (probability) is lower.
    Bob S
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