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The relation between atoms' kinetic energy and the energy levels

  1. Mar 27, 2013 #1
    Regarding the relation between atoms' kinetic energy and the energy levels of their electrons upon excitation:

    In other words, what really happens when an atom is excited, either by radiation or by collisions, or otherwise ? What are the mechanisms under which the transferred energy goes to exciting the electronic levels, the atom's kinetic energy as a whole, or both ? in the latter case, which is supposedly the most common case, what's the ratio between the two (elec energy levels & atomic kinetic energy) ? Any elaboration on these aspects will be great.

    Thank you, BC
  2. jcsd
  3. Mar 27, 2013 #2
    since the energy transfered has to be the difference between some energy levels the difference with the energy of the radiation or the colliding particle is probably transmitted to the kinetic energy of the whole atom.
  4. Mar 27, 2013 #3
    A collision can only result in a vibrational or rotational energy level transition. This is a transfer of kinetic energy. I would speculate (so don't quote me on this) that it would require extremely large amounts of thermal energy to cause an electronic transition as a result of kinetic energy transfer.

    By absorbing a photon of near-UV/Vis light, a substance can undergo an electronic transition. When this happens, electrons are promoted to a higher energy level. Between electronic energy levels there are vibrational energy levels, and between vibrational energy levels there are rotational energy levels, and all are quantized. The molecule/particle must undergo nonradiative vibrational transitions in the form of collisions before its promoted electron can "relax" to its ground state. When the electron relaxes, the difference in energy between the excited and ground states is released in the form of a photon. The energy of the emitted photon is always less than the energy of the absorbed photon, due to the nonradiative transitions that occur.

    I'm only an undergrad so I hope this helps!
  5. Mar 27, 2013 #4


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    Staff: Mentor

    You sure about that RedDanger? I'm nearly certain that thermal motion can cause electronic transitions. How else would hot gas emit radiation?
  6. Mar 27, 2013 #5
    I'm assuming that by "hot gas" you're referring to a plasma, in which case I'm fairly certain that a large amount of thermal energy is supplied. For example, consider an ICP laser that reaches temperatures of up to 10000K. I can't think of another type of hot gas which would emit in the Vis region.

    Another instance I was thinking of would be extremely hot metals, like Iron, which can turn white-hot (it's white because they're emitting Vis-region photons), or blackbody radiation. In any case, thermal energy can cause radiative emissions, but not necessarily in the Vis region of the EMR spectrum. At least, that's my understanding.
  7. Mar 28, 2013 #6


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    Staff: Mentor

    If that were true, then atoms, which only have electronic internal degrees of freedom, would never emit radiation when heated.

    It can undergo nonradiative transitions, but there is no reason why it must.
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