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Large Static E Field, Dielectric Break Down, Quantum View

  1. Feb 1, 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 ......




    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?




    Still a little uncertain.

    Thank you all for your responses.

    Best

    Az
     
  2. jcsd
  3. Feb 5, 2010 #2

    Claude Bile

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    Your supposition is wrong. You can get ionisation through non-resonant excitation. Radio sources will induce ionisation at sufficient intesities.

    Are you familiar with Keldysh theory? This is the theoretical framework that governs the transition between tunneling ionisation and multi-photon ionisation in the non-resonant case.

    Claude.
     
  4. Feb 5, 2010 #3

    ZapperZ

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    This is a bit confusing.

    Let's start first of all by saying that in an electrical breakdown, there's a "source" of ionization. This is true be it in a static or alternating E-field.

    Secondly, the external E-field is significantly lower than the ionizing potential of the molecules involved.

    In many instances, there is an "initiator" or source. Often, it is a region with high field-enhancement (sharp, pointy objects). The most accepted explanation involves the formation of field-emission current (see Fowler-Nordheim model) from these field-enhancement region. These currents gain significant energy from the external field so much so that they can collide and ionize with the surrounding gas. This in turn can often lead to a cascading effect resulting in the formation of a plasma for a very brief period (see this: http://focus.aps.org/story/v19/st4).

    The "quantum" aspect in all of this is not really in the field itself, but in how the source or initiator react to the field. The Fowler-Nordheim model invokes tunneling effects that resulted in the field emission current.

    Zz.
     
  5. Feb 8, 2010 #4
    Thank you

    Soo, I'm assuming that the ionization due to high electric field occurs because of some (initiation), like how water begins to boil (spontaneous boiling)

    I still maybe wrong.

    Another thing, according to this Keldysh theory (previous reply)

    you can ionize atoms (strip off electrons) with extremely high intensity radio waves.

    Am I correct in saying that this ionization would not occur (if it was not for tunneling)?


    Thanks again everyone
     
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