Momentum exchange during diffraction

  • #1
I just learnt that there is a well established theoretical picture that explains the change of momentum associated with direction change during a diffraction event, in terms of quantized momentum transfer to the diffracting object. For example, https://en.wikipedia.org/wiki/Duane's_hypothesis

This picture is also supposed to be applicable to diffraction during transmission through a single slit or a double slit.

I am just wondering what kind of forces actually mediate the exchange, e.g. in the cases of electron diffraction and photon diffraction. I found hints suggesting that this could be related to phonons within the screen material, but - if so - wouldn't the diffraction then be a function of the solid state physics happening inside the screen?

This leads to my actual question : is it reasonable that the force that "mediates" the deflection and momentum exchange is a kind of Casimir effect occuring within the slit volume, where certain EM modes are suppressed by geometry?

Also, what force would mediate the momentum transfer when a photon is diffracted through a slit?
 

Answers and Replies

  • #2
Simon Bridge
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what kind of forces actually mediate the exchange
You can work it out: there are only four fundamental forces ... list them and see which makes sense. Consider: how do photons normally interact with matter?
The specifics of the interaction are very complicated - it's a bit like how a collision is very complicated but you can do reliable physics on the bits that fly away by using conservation of momentum if we use a "before" and "after" approach.

is it reasonable that the force that "mediates" the deflection and momentum exchange is a kind of Casimir effect occuring within the slit volume, where certain EM modes are suppressed by geometry?
... only by analogy. The possible paths between source and detector are open - the rest supressed. This is not a casimir effect though, just normal absorbtion etc.

Also, what force would mediate the momentum transfer when a photon is diffracted through a slit?
You can work it out: you have a choice of four. List them: which one applies.

See also:
http://iopscience.iop.org/article/10.1088/0143-0807/23/6/303/meta
... gives you an idea about what sort of momentum exchange would be present.

Make sure you see the followup though:
http://iopscience.iop.org/article/10.1088/0143-0807/32/1/010/pdf

... full text also available through arxiv.

And just in case:
http://www.vega.org.uk/video/subseries/8
... accessible description of photon-matter interactions.

 
  • #3
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My understanding is that the phonon model for crystal diffraction constrains the transfer of momentum as energy must also be conserved. It is the lattice spacing that is important.
 
  • #4
Thanks. I'll look at those references.
 

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