What Happens When a Photon Meets an Isolated Electron?

[gareth]

Here's a thought experiment;

Suppose we could isolate one single electron in space and there are no external forces acting upon it (I don't think this is even possible, is it?)

and we fired a single photon at the said electron, which direction would the electron move in?

here's my thinking; the alternating EM field will cause the electron to jiggle a little bit, but as the interaction ceases, the electron will travel in the last direction it was traveling prior to the end of the interaction, i.e in a transverse direction to the incoming photon.

...but, the photon also has momentum right? so if it hits the electron head on would we see the electron move in the direction the photon was travelling?

any thoughts?

 

[ZapperZ]

Why is this not Compton scattering?

Zz.

 

[gareth]

Why is this not Compton scattering?

Zz.

Not quite, I'm wondering if the photon is completely absorbed by the electron, what will happen.

 

[uart]

Not quite, I'm wondering if the photon is completely absorbed by the electron, what will happen.

I don't think it could satisfy both conservation of energy and conservation of momentum if the photon was completely absorbed and no photon re-emitted. I think it would also require the emitting of a lower energy photon. Someone refresh my memory, is that what Compton scattering is?

 

[ZapperZ]

Not quite, I'm wondering if the photon is completely absorbed by the electron, what will happen.

How is a free electron able to do that? What happened to the conservation of spin?

Zz.

 

[uart]

How is a free electron able to do that? What happened to the conservation of spin?

Zz.

Interesting, I was thinking of momentum and energy and didn't even think about spin. So the photon can get shifted in energy (frequency) but there must still be a photon after the collision.

One question Zapper. What happens to spin in the photolectric effect, where a photon is fully absorbed and an electron ejected?

 

[ZapperZ]

Interesting, I was thinking of momentum and energy and didn't even think about spin. So the photon can get shifted in energy (frequency) but there must still be a photon after the collision.

One question Zapper. What happens to spin in the photolectric effect, where a photon is fully absorbed and an electron ejected?

The photoelectric effect requires the whole solid to be present. So any momentum conservation (and spin conservation) laws are taken up by the lattice.

Zz.

 

[gareth]

How is a free electron able to do that? What happened to the conservation of spin?

Zz.

I don't know, please elaborate on the conservation of spin.

 

[Vanadium 50]

How is a free electron able to do that? What happened to the conservation of spin?

Why is spin a problem? Energy and momentum conservation prevents this from happening, but I think it's okay with spin, provided the photon flips the spin of the electron.

 

[ZapperZ]

Why is spin a problem? Energy and momentum conservation prevents this from happening, but I think it's okay with spin, provided the photon flips the spin of the electron.

Er.. but can this spin flip actually occur when you have an electron not in any spin state?

Zz.

 

[Vanadium 50]

Er.. but can this spin flip actually occur when you have an electron not in any spin state?

Well, it's in some state.

Even if I have an unpolarized electron and an unpolarized photon, the system could be in either a state of total spin 3/2 or 1/2. My only point is that only the 1/2 state allows for photon absorption.

 

[ZapperZ]

Well, it's in some state.

Even if I have an unpolarized electron and an unpolarized photon, the system could be in either a state of total spin 3/2 or 1/2. My only point is that only the 1/2 state allows for photon absorption.

But since this is not in any magnetic field, the states are still degenerate and with no energy difference. So there is no energy absorption corresponding to a spin flip state.

Zz.