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Black Body Molecular Absorption

  1. Dec 5, 2014 #1
    I understand photon absorption/emission by electrons in atoms, that transition them between energy levels, producing sharp spectral lines. However, I am having difficulty understanding photon absorption/emission by the vibrational and rotational modes of molecules, with respect to black body radiation. In this case the spectrum is continuous, suggesting that the energy levels are all close together. What is it exactly within the molecules, that absorbs the photons? Is it:
    1) The electrons again, this time with the same discrete energy levels, but broadened by the Doppler-shift of the vibrating/rotating molecules?
    2) The electrons, with energy levels, unrelated to the atomic spectral lines?
    3) Some other mechanism that doesn't directly involve electrons?
  2. jcsd
  3. Dec 5, 2014 #2


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    Vibrational and rotational modes aren't really continuous; they just have closely spaced spectral lines. However, if the Doppler broadening is larger than the spacing between the lines, then the spectrum is continuous for all practical purposes.

    The entire molecule absorbs/emits the photons. To understand this, consider the simpler case of transition lines in a hydrogen atom. These are mostly due to electron transitions, but remember that in a two body system, both the electron and proton orbit the center of mass. The hydrogen energy levels involve the reduced mass of the electron and proton, which only differs slightly from the electron mass due to the large difference in masses. So we usually attribute the photon emission due to the electron motion, which is mostly true. But the proton contributes a little bit. It's more correct to say the atom absorbs/emits as a whole.
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