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Atom Excitation: Absorption Peaks or short wavelengths?

  1. Sep 15, 2009 #1
    I just have a quick question.

    Which way of Atom Excitation is more efficient?

    Photon Energy which matches the absorption peak(s)?

    Or Photon energy of a Shoter wavelength than the absorbtion Peak?
  2. jcsd
  3. Sep 15, 2009 #2
    Photon with a "shorter wavelength" than necessary will not "hit" the right spot... You'll surely see a response (just like you can pluck a guitar string with any frequency you'd like) but it won't be as "efficient"..

    So the "resonant wavelength" will do a better job - like in many cases in quantum systems.
  4. Sep 15, 2009 #3
    Can you describe "Resonant Wavelength"???

    Does the resonant wavelength of an Atom change when it's excited or ionized?

    What about if the atom is in an electrical field? What effect does that have?
    Last edited: Sep 15, 2009
  5. Sep 16, 2009 #4
    Resonant wavelength is the wavelength of the electromagnetic radiation that exactly matches the difference of a pair of discrete energy levels of an atom...

    It doesn't change for an excited atom, because an excited atoms discrete energy levels (approximately) do not change when an electron is excited.

    If it's ionized however, you change the electrostatic balance between the nucleus and the electrons, therefore the exact placement of energy levels might change significantly.
    For this reason, the resonant wavelength also changes.

    Similarly an electric field changes the discrete spectra , time independent "perturbation theory" allows you to calculate the differences in the spectra provided that the electric field you apply is small compared to the electric fields already existing between the nucleus and the electrons.
  6. Sep 16, 2009 #5


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    There's no reason 'resonance' has to apply to electronic levels only. (NMR, EPR, etc)
  7. Sep 20, 2009 #6
    Hi HMS-
    A very good example of this resonant absorption is the 3P-->3S sodium spectrum (the yellow doublet).
    If you look at the lines with a diffraction grating you will see the yellow doublet (wavelength about 5890 Angsroms)
    with about a 5 Angstrom splitting. Each of the two lines is broadened by the temperature of the sodium gas in the arc. If you look carefully, you will see a dark absorption line in the middle of each doublet line, caused by the emission lines being strongly resonantly absorbed by the surrounding colder sodium gas.
    Bob S.
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