Electron Diffraction - Part 2

This is a demonstration showing the wave-like properties of electrons using diffraction from a graphite powder.

Video posted for use under Creative Commons Licence - by-nc-sa

Category:

Science & Technology

Tags:

• Physics
• UNSWTV

[freemanx2x]

Why the interference pattern looks like the diffraction made by a single small hole? Shouldn't the electron-wave be diffracted by all the atoms in the graphite? I would expect that by increasing the voltage to get an interference pattern close to the X-ray diffraction pattern in crystals...How is tat?

[micolich]

@freemanx2x In short, its because the material is polycrystalline. The target is a copper grid with graphite deposited on it, that graphite consists of an agglomeration of tiny crystalline graphite domains (hence polycrystalline). Each of these domains has an identical crystal structure, but they are randomly oriented with respect to one another.

A large number of these sit within the cross-sectional area of the incident electron beam. Some will have an orientation such that the incident...

[micolich]

@micolich ... such that the incident beam comes in at the Bragg angle, and this produces strong diffraction out to the first ring that you see. The second ring corresponds to the 2nd order situation for above. The domains where the electron beam doesn't come in at the Bragg angle destructively diffract, so these don't blur the signal.

If you then think through all the possible ways you can orient the diffracting domains so that Bragg's law holds (you might need to draw this up), you get a ring.

[micolich]

@micolich For more info, try looking up the 'powder crystalline diffraction method', I think it was developed by Debye and ummm, Scherrer (or maybe its von Laue). You are right though, if you had a pure single crystal in there, you'd get an x-ray like diffraction pattern. Try looking at wikipedia's electron diffraction page for an image of this (youTube won't let me post the link).

[micolich]

@micolich Very nice question by the way!

[adam3141]

I have done this experiment in my Physics class a month ago and it is pretty cool but it also got me thinking and I know that I could be totally wrong but I have a few questions.

* Could the electron be radiating EM radiation that is causing the interference pattern?

* Has this experiment ever been done with neutrons?

* Does the 'matter wave' travel at the electron speed or the speed of light?

* Did the electron undergo quantum teleportation?

[micolich]

@adam3141 Q1: No, because the light is emitted when the electron reaches the phosphor, so it does map where the electron hits the screen. Q2: Yes, it has, it has even been done recently with C60 (buckyballs). Q3: The speed of travel is a difficult one, I need many more characters to answer. Q4: No, the electron did not undergo quantum teleportation.

[tobywharne]

Great stuff!

[coburgmaths]

Excellent vid Adam. Cheers.

[freemanx2x]

@micolich Thank you for your detailed answer!

(wwwyoutubecom/watch?v=kTWwtJj­hI-0&feature=related)

[lssam100]

Thanks a lot for this great efforts.

[moranginka]

Excellent video! Thank you. You explained it perfectly.