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Continuous absorption

  1. Sep 12, 2011 #1
    hi

    i was thinking that after the laws of quantum mechanics, where all atoms and molecules have discrete energy states, things like continuous spectrums should be forbidden. but why are there still molecules like chlorophyl which show this property? i would think, that this means, that the energy levels of chlorophyl are infinitely close together, but i guess this can't be right?
     
  2. jcsd
  3. Sep 12, 2011 #2

    Ken G

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    Gold Member

    The problem is that the "energy levels" in an atom are themselves an idealization. Real atoms don't have stationary states, because they live in an environment of constant perturbation. Even if you isolate the atom, you cannot isolate it from its capability to generate a photon if it is in an excited state-- and that capability, via vacuum interactions, is enough to get the atom to perturb itself, in effect. So that's why so-called "stationary" excited states can still make transitions (and have a finite lifetime), but along with that comes the fact that there is some uncertainty, or spread, in the energy these transitions can absorb and emit. If the transitions are separated widely enough, we might ignore that spread and make statements like "atoms can only absorb and emit at special frequencies", but this is never formally true. And when the possible transitions get so densely packed in complex collections of atoms that their spread starts to overlap, then you get a continuum source. High density can do this in gases, but very complex molecules achieve some of the same things (though I'm more familiar with molecular "bands" than with what you are calling the continuum of chlorophyll).
     
  4. Sep 12, 2011 #3

    Bill_K

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    A familiar situation occurs in a metal, where the conduction electrons delocalize, being shared by many atoms. This allows for many closely spaced energy levels forming a conduction band.

    A similar situation occurs on a smaller scale in pigments such as chlorophyll, which contain a conjugated system such as a porphyrin ring, made of alternating single and double bonds that allow the electrons to delocalize. The result is a set of molecular orbitals having closely spaced energy levels.
     
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