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Static Electric Fields / virtual photons / Ionization

  1. Jan 31, 2010 #1
    Hi

    Got a question, hoping someone can give a good explanation for.

    We all know that large static uniform electric fields have the potential to ionize. (Dielectric breakdown of Air - Static Electricity Shock).

    In the Quantum picture, why does this occur. Take for instance the photo electric effect, increasing the intensity of radiation does not allow for electron ejection/excitation unless the frequency is above a certain threshold.

    The frequency for a static field, I'm assuming to be zero. I'm thinking that, (virtual photons) could play a role in the underlying static field, and these photons might be the right frequency to ionize?

    For a large static field, I understand why the ionization occurs classically but ......


    Please, enlighten me.

    Thank you.

    Az
     
  2. jcsd
  3. Jan 31, 2010 #2

    SpectraCat

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    Science Advisor

    Actually, all you need for your example is a classical field large enough to compete with the electric field felt by electrons near the nucleus (~ 1 MV/cm or so). When this linear static field is added to the coulomb potential, the summed potential has a barrier on one side of the nucleus, and so the electrons have a finite chance of tunneling out into the unbound continuum. It's hard to explain in words, but perhaps the attached figure (from the website of the Ditmire group at UT-Austin) makes it clearer.

    Note that although the explanation is for a laser field, it is equally valid for a static field. Note also that if the field is made large enough, the electrons will become classically unbound, and won't even need to tunnel, although in truth I don't know if such large fields could be created.

    One other important point is that discharges often start from asperities, which are microscopic high aspect ratio regions which act as field line collectors. In other words, discharges happen much more easily when a field-gradient is present, but they are still possible in homogeneous static electric fields.
     

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  4. Jan 31, 2010 #3
    Still a little uncertain.

    I understand that a large enough static field, can liberate the electron classically.

    I understand that even without this large static field that tunneling is a possibility.

    What I don't understand is how a static field is really that much different from extremely low frequency radiation.

    What I have learned is that only certain frequencies of radiation (light) can produce excitation or ionization. Increasing the intensity of a non resonant frequency will not liberate the electron (here the frequency is less that than threshold Like photo electric effect).

    For instance, take really large intensity radiowaves Such that they will produce a huge electric field that is time varying. But they should not be able to ionize an atom?

    Is the difference here that large intensities are many single photon combination.
    What is so different between a high intensity static electric field and high intensity extremely low frequency radiation, other than the magnetic field tagging along with the radiation and the sinusoidal dependence?

    Thank you all for your responses.
     
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