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Another photoelectric effect related question

  1. Jun 19, 2008 #1

    After searching the forum I did not find an answer to my question, so here goes.

    In the classical theory a photon can only eject a photo electron when the energy of the photon is greater than the binding energy of the electron. This is only possible with bound electrons as some of the recoil momentum has to be taken over by the nucleus.

    The photoelectric effect was observed with visible light and a metal. I assume that the photo electrons emitted from a metal by visible light are actually electrons originating from the conductance band (CB) as the photon energy is quite low. This also means that the recoil momentum is actually taken over by the crystal lattice of the metal. Is this correct?

    When the incident photons far greater energy and we still try to eject photoelectrons from a metal what will happen? I think that no electrons from the CB will be ejected and, depending on the energy, bound electrons will be ejected because the recoil momentum can now only be taken over by the nucleus. Or will there also be electrons emitted from the CB (now with far greater kinetic energy)? Is related to the atomic cross-section of the photoelectric effect?

    Someone with some input?
  2. jcsd
  3. Jun 19, 2008 #2


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    First of all, the "photoelectric effect" doesn't just apply to visible light. One can also do this with UV sources, and in fact, that's what you will need if you want to see this effect on something like copper, which has a work function of around 4.7 eV.

    When the photons have energy far greater than the work function, a number of things can happen. You can still get electrons from the top of the conduction band (near the Fermi level), and these electrons will be liberated with a lot of kinetic energy. For example, look at a typical photoemission spectrum. You'll see a plenty of electrons at the high energy tail end of the spectrum. You can also start getting photoelectrons that came deeper from the conduction band.

    Beyond that, things start to get complicated. Depending on the skin depth of that particular photon frequency and the material, one can also start to get core-level photoemission. This is where the photoelectrons came not from the conduction band, but from the bound state of the atoms/molecules that made up the material. One can also start to get Auger processes where this is now no longer a "photoemission" process.

    And yes, the recoil momentum is taken up by the lattice.

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