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Photon Absorption wavelength(s) to aid ionization?

  1. Sep 19, 2009 #1
    If I have a group of atoms in a high voltage field, and I want to use photon energy to aid the ionization of the atoms what wavelength(s) would best be used to have the highest ionization efficiency?

    The shortest wavelength available?

    Or the resonant wavelength for each excitation level?
  2. jcsd
  3. Sep 19, 2009 #2
    If you want to start a discharge with help of initial "ignition" with photons, there might be different possibilities:

    1) You excite atoms with available source of resonance photons and the high-voltage electric field ionizes more easily the excited atoms,

    2) You use photons with energies a little bit higher than the ionization energy (the maximum of photo-ionization is reached not at Eptoton = Eionization but at higher energies),

    3) You use X-rays for that.
  4. Sep 19, 2009 #3
    Good answer!

    Lets say I only have 2 choices:

    1. I can use the resonant excitation wavelengths (we'll say they are all longer than 450nm)

    2. Or i can use UV at 370nm

    Wouldn't the UV contrubute more to the ionization than the resonant excitation wavelengths since the UV is shorter (higher energy)?
  5. Sep 19, 2009 #4
    Yes, I think so, especially in presence of an external electric field. It is known that the light from electric arc makes charged body discharge due to air ionization with UV light.
  6. Sep 19, 2009 #5
    Most individual atom photoionization cross sections are in the 10 Mb (megabarn)(1 Mb = 1 x 10^-18 cm^2) range. See
    Usually, the cross section is maximum at threshold, but sometimes it may increase slightly a few eV above threshold. In most cases, the cross section is nearly a factor of 10 lower by 50 eV.
    [Edit] You might find this table of elements sorted by ionization energy handy:
    Bob S
    Last edited: Sep 19, 2009
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