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Direction of Photo Electron Emission

by Leb
Tags: photoeffect
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Jun16-14, 03:31 AM
P: 94
I was looking for information on how the photo electrons are emitted when under X-ray radiation. In this ancient review paper here they state that the most common angles for non polarized X-ray beams (of various energies) range roughly at around 70-80 degrees with the beam. It is unclear to me, whether the photo electrons are moving towards the source of the X-ray beam or away from it ? Undergrad texts do not seem to shed light on this matter, the best one gets is pictures with emitted electrons being at a 90 degree angle to the incoming photon. Also, I assume this angle is given for a cone, i.e. it's 70-80 w.r.t. the beam, but with 2pi angle around the beam ?
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Jun16-14, 09:05 AM
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PF Gold
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Debends upon whether the beam is incident from the front or the back. I've done a lot of photo-electron generation, mostly in transmission.

The energy reqired to emit an electron is the "work function", which varies with the crystal plane that is encountered: 111, 210, etc. Each has a slightly different work function.

But the liberated electron may shoot off in any most any direction, though there are statistics (which I don't recall the details at the moment; they are in my old notes).

Most interactions don't make it out because of (a) going in a bad direction, or (b) scattering. Thus only interactions within a skin depth might be productive, and even then not many.

If there is an extraction field, the electron paths get straightened out.
Jun16-14, 02:09 PM
P: 94
Well, that "any direction" answers seems to conflict with experiments from 1920-30's, I wonder why did they find a preferential angle...

M Quack
Jun18-14, 09:07 AM
P: 662
Direction of Photo Electron Emission

There is actually some information that can be extracted from the direction of emission. The technique is called
ARPES (angle-resolved photo-emission spectroscopy)
Jun18-14, 09:30 AM
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Quote Quote by M Quack View Post
There is actually some information that can be extracted from the direction of emission. The technique is called
ARPES (angle-resolved photo-emission spectroscopy)
Since I had done ARPES, I can comment on this.

The OP asked about x-ray photoemission, which is distinctly different than the "standard" ARPES experiment that is done in the UV range.

Secondly, in ARPES, the preferred direction of emission is due to the in-plane momentum of the electrons in the surface, i.e. the "k" in band structure. However, this also means that one must perform the experiment on single-crystal material. A polycrystalline or amorphous crystal will not have such directional emission and all you'll get is a momentum-averaged density of states.

I have not looked at the OP's reference, and thus, don't know if there's something similar here. XPS has other factors involved, and any preferential direction of photoelectron emission can be due to other reasons than what I stated here. Certainly, it could be angle-resolved XPS, which would have the same explanation as what I've given above.


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