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Oscillation of one molecule or atom

  1. Sep 17, 2012 #1
    Hello all,

    Many Physics texts simply say that atoms "vibrate" when heat energy is transfer to a metal bar when it is heated. Or that molecules vibrate as the result of heat transfer.
    I'm trying to understand what makes an atom or molecule "move" or oscillate when energy is given to it.
    If I have only one molecule in a region of space with no other molecules or atoms around it and a beam of light from the sun hits it, the molecule will oscillate up and down because of the electromagnetic fields? It is just a matter of electric charges in the molecule moving in a field?

    I don't think the atoms would absorb energy and start to move by themselves as if something inside the particles were creating that motion like a mexican jumping bean. Either they move as a result of momentum being transferred or by the charges being submitted to an electromagnetic field (or even because of a gravitational field acting upon its mass)

    In brownian motion we know that the molecules move as the result of numerous collisions they undertake one another. But what if I have only one molecule?

    many thanks for any comments.


  2. jcsd
  3. Sep 17, 2012 #2
    Oscillations or vibrations occur under the control of a restoring force.
    They cannot occur without this second agent.

    Have you done simple harmonic motion?

    In an aggregate of atoms the restoring force is provided by interatomic forces.

    A single isolated atom cannot vibrate, mechanically.

    It can, however be involved in collisions with other particles and suffer change of momentum.
    Photons have momentum.
    However they are so small they are will affect individual parts of the atom rather than the atom as a whole, particularly the outer electrons.

    Atoms are electrically neutral so in general they can't interact with electromagnetic fields.
    However they also have physical size and they are neutral because they contain an equal number of positive and negative charges.
    These charges are not all in one place but are distributed within the atom.
    This leads to the atom being able to act like a dipole or quadrupole and participate in vibrations in EM fields. This is how NMR works.

    Does this help?
  4. Sep 17, 2012 #3
    It will oscillate if it's charged. A neutral molecule may also rotate or vibrate internally if it has an electric dipole change associated with that movement.
  5. Sep 17, 2012 #4
    Hi Studiot,

    thanks, it helps.

  6. Sep 17, 2012 #5
    Atoms cannot vibrate alone. They also cannot rotate. In chemistry atoms are treated as point particles for purposes of rotational and vibrational spectroscopy.

    However molecules can rotate and vibrate; when they vibrate at their normal modes, they can emit radiation (usually in the IR band) at the frequency of these normal modes.
  7. Sep 18, 2012 #6
    How can a single atom NOT rotate alone? that doesn't make much sense, ofcourse for mathematical purposes a rotating single atom may be the same as a still one(apart from... magnetism?) but if you shoot light at it from an angle, it should using common sense, be shocked away in a spinning momentum, assuming it reflects the light.
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