Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

IR from atoms

  1. Apr 30, 2012 #1
    Hello, why do every atom with a temperature different from the absolute 0 emittes infrared radiation? Where does it came from?

    Thank you!
  2. jcsd
  3. Apr 30, 2012 #2
    It comes from the quantized vibrational modes (Stretching, Wagging, ...). Each mode can be regarded as a harmonic oscillator with qauntized energy levels. The difference in energy between two successive levels is such that absroption and emission are in the infra red regime of the spectrum of electromagnetic radiation.
  4. May 6, 2012 #3
    So infrared radiation is just energy, part of the atomic energy?
  5. May 8, 2012 #4
    I'm not sure I understand this. I'd say that Infra Red absorption/emission is the outcome of vibrational modes excitation/d-excitation
  6. May 8, 2012 #5
    is this true?

    In chemistry, IR spectra show bonds, not individual atoms...

    Like Useful nucleus said...the wagging, etc. of the bonds.
  7. May 13, 2012 #6
    you mean that a single atom doesn't emittes IR radiation but there must be some bond?

    thank you
  8. May 13, 2012 #7
    I think we are talking about different things.

    To help determine the identity of an unknown chemical, we can do an IR analysis. We shine IR radiation through a chemical, and the bonds between atoms will absorb IR radiation at specific wavelengths.

    I don't know about atoms giving off IR radiation unless maybe it is from an electron moving down from an excited state.
  9. May 14, 2012 #8
    I think probably you are asking about thermal emission of radiation due to interatomic collisions, not resonant emission and absorption of radiation due to electron transitions.

    This is actually not so simple I think, and I am not 100% sure of the how this mechanism really works. I can describe for you what I think must be similar to what happens, and perhaps someone else can clear up the details.

    So first, an individual atom does not have a temperature, only a collection of them does, and the radiation you speak of is generated because of the collisions between these atoms. I assume the initial generation of photons during these collisions must be due to bremstrahlung or something, or maybe just off-resonance transitions, (one way or another some of the atom kinetic energy gets converted into photons) and if you are in a thermal bath of atoms then loads of these photons will be created. These photons then interact further with atoms other than those which emitted them, and even though most of them are way off resonance (i.e. are not likely to kick electrons up and down in their orbitals) interactions can still happen with low probability, and since there are so many atoms and photons this happens a lot. Momentum can be transferred back and forth from the photon bath to the atom bath in this way, thermalising them (so they have the same average energy), and so your final photon spectrum will match the thermal spectrum of your atoms.
  10. Jun 13, 2012 #9
    So a single atom will have no emission of any kind?
    If I take for example 1 g of Fe and I heat it up to 500 °C, then I take 0.1 g Fe at the same temperature will they have the same emission?
  11. Jun 13, 2012 #10
    Not if it just sits there by itself, not interacting with anything. Of course it emits if its electrons can get some energy from somewhere and do some jumping around.

    They will have the same emission spectrum. I am not sure how this relates to the first part of the question though. There are vast numbers of atoms in both 1g and 0.1g of Fe. Avogadros number is of the order 10^26 after all (for Fe this is the number of atoms in 56g of the stuff).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook