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Metals, X-rays, and plasma frequency

  1. Apr 15, 2006 #1

    pervect

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    I was wondering if it would be fair to say that the frequency at which x-rays start to penetrate a metal would be the plasma frequency associated with the electron density of the metal, i.e. the frequency given by

    http://scienceworld.wolfram.com/physics/PlasmaFrequency.html

    or whether some other mechanism was involved.
     
  2. jcsd
  3. Apr 17, 2006 #2

    Tide

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    Have you tried calculating the electron plasma frequency for some typical metals?
     
  4. Apr 17, 2006 #3
    you wouldn't be correct. plasmon energies are several orders of magnitude lower than the x-ray range.
     
    Last edited: Apr 17, 2006
  5. Apr 17, 2006 #4

    pervect

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    Using the MKS formula at

    http://www.everything2.com/index.pl?node_id=1529021

    and using N = 8.47 * 10^28 / m^3 for copper

    http://hyperphysics.phy-astr.gsu.edu/HBASE/tables/fermi.html#c2

    I am currently getting 2.6*10^15 hz

    This seems to probably be a bit low, in the UV range. But I don't know what the effective mass of an electron in copper should be, nor do I know what the permittivity value should be - they probably shouldn't be the free-space values I used for both.

    I also don't really know when copper starts transmitting x-rays, for that matter :-(.
     
  6. Apr 17, 2006 #5

    pervect

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    What's a plasmon?

    It looks like the estimate in terms of plasma frequency misses the mark by a few orders of magnitude from remarks that have been made - any ideas of how can it be "fixed up" to get within, say, one order of magnitude?
     
  7. Apr 17, 2006 #6
    plasmons are charge density fluctuations (collective excitations) in metals. same stuff but treated quantum mechanically taking the proper electronic structure into account. I don't think you can get much closer since in a lot of metals the electron effective masses aren't too far from the bare electron mass.
     
  8. Apr 17, 2006 #7

    Gokul43201

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    If you are trying to calculate the frequency at which a metal starts to become transparent to radiation, that would be the plasma frequency. Most metals are transparent at high UV frequencies.

    So, to correct the statement in your OP :
    ...it would be fair to say that the frequency at which EM radiation starts to penetrate a metal would be the plasma frequency associated with the electron density of the metal...
     
  9. Apr 18, 2006 #8

    pervect

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    OK, that answers the question, thanks. And it nicely explains why the semiconductor people have trouble building UV mirrors, too.
     
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