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Why the photoelectric absorption section finite at threshold

  1. Apr 12, 2015 #1
    I mean the photoelectric effect of the hydrogen atom.

    It is weird. By the Fermi golden rule, the transition or absorption rate is proportional to the density of the final states. At threshold, the electron has zero momentum and thus zero density of state. Therefore, the absorption coefficient should be zero at the threshold. Where does this argument go wrong?
  2. jcsd
  3. Apr 12, 2015 #2


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    Exactly zero momentum is not a proper state. Every finite transition time gives a small momentum range.
  4. Apr 12, 2015 #3
    anyway, close to the threshold, the absorption section should be close to zero. But in reality, it is finite.
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