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Heating with photons

  1. May 10, 2005 #1
    Can someone explain to me how the photon energy can heat up a material. Shouldn't the photon energy cause interactions(photoelectric, compton etc.) How can this energy be "absorbed" to make atoms move faster?
    Last edited: May 10, 2005
  2. jcsd
  3. May 10, 2005 #2


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    Molecular translation and vibration spectra are typically in IR.The material absorbs the photons of all frequencies and emits them in this short band.The rest of the energy is converted into phonons,in the case of solids.

  4. May 10, 2005 #3
    How real are phonons? At the risk of sounding heretical, is there any justification for a "particle of sound" model other than its resemblance to other quantum particle models? Be sure, I am not questioning the mechanics of this model.
  5. May 10, 2005 #4
    Phonons can be observed in neutron scattering experiments for example. "particle of sound" sounds bad. Quanta of lattice vibration sounds better to me.
  6. May 10, 2005 #5


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    Then you need to define what you consider as "real".

    Phonons are collective mode excitation. In that sense, they aren't "real" because they do not exist when you take the system apart. However, they are "real" in the sense that they become a mathematical tool for interactions. You can replace the complicated EM interactions of the ions by the field theoretic approach of particle exchange, and not only get the same answer, but also gives you insight into OTHER phenomena.

    What I have stated is not restricted to just phonons, but also to other collective excitations in matter, such as magnons, polarons, spinons, etc.

  7. May 10, 2005 #6
    Yes, the word 'real' is a wee bit vague. I'll ask different questions:

    1. Is the phonon described in the standard model?

    2. Does the phonon model describe anything that photonic interactions cannot?

    As I understand it, an electron can become seemingly positively charged by phonon interactions. So...

    3. Do phonons carry charge?

    4. Could the same phenomenon be described by the charge of an electron being obfuscated by the greater total of positive charges from nearby ions?

    I'd forgotten all about phonons. I'd be interested in reading more. Do you know of any links suitable for an undergrad? Much obliged.
  8. May 10, 2005 #7


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    It appears that you have a rather misleading picture of what a phonon is. A phonon is not part of the Standard Model because the Standard Model is a model for "fundamental particle". A phonon, as I've said, is a collective excitation of lattice vibration in solids.

    While EM interactions/photons are responsible for the binding force between ions and electrons in a solid, when there's a gazillion of them, it is impossible and impractical to account for each individual interactions (you'd never be able to explain anything this way). Thus, when the lattice vibration interacts to cause a phenomenon (such as metallic resistivity, conventional superconductivity), one can easily use QFT and treat phonons as the "force exchange carriers" the way one would normally use other force carriers.

    Any solid state texts such as Kittel or Ashcroft-Mermin has ample introduction to phonons, magnons, etc.

  9. May 10, 2005 #8
    Ahhhh, the penny drops. Yes, you are right. I have read about things emitting and absorbing phonons, and it did leave me with the impression it was a new fundemental particle - that qualification was not clear. You have confirmed what I previously thought, though - it is essentially a simplification of undescribably complex and numerous EM interactions. I am still a little unsure of the application of this analogy though.

    When you say the phonon is "a collective excitation of lattice vibration in solids", do you mean the entire lattice (which, I think, could be the entire solid), or just an area of it? Or is it ad-hoc? That is, you could describe the excitation of a single atom as a phonon or, say, an entire electrical wire as required?

  10. May 10, 2005 #9


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    By definition, a "collective" phenomenon requires more than just "a few" participants. So "one" single atom will not cut it, not even close.

    I've said already that if you take the system apart, a phonon doesn't exist. Whether the entire bulk material or the entire lattice is involved depends on the extent of the lattice. In principle, if you have a single crystal, the phonon modes are defined over the entire single crystal lattice. It isn't localized over a particular region in real space.

    Again, this thing will become apparent once you try solving the normal mode vibrations in the simplest case - the 1D ions.

  11. May 10, 2005 #10
    Thanks. I think I have the basic concept.
  12. May 10, 2005 #11
    The photons collide with atoms and are either absorbed or scattered. Each of which transfers kinetic energy to the atoms which increases the kinetic energy of the particles. This shows up as heat.

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