Can electrons absorb a photon?

[Dale]

The potential energy is a property of the atom, not a property of anyone single component of the atom.

This is an important point that applies for potential energy in general. We sometimes speak as though it is localized to one part of a system, but generally such comments are misleading.

 

[DanMP]

... Did you ever sit down and work out the conservation of momentum and energy as I recommended?

If so, then you should have seen that scattering can conserve both energy and momentum with just the electron, but absorption cannot. That is why a single electron can scatter a photon but cannot absorb it. It is all about conservation principles. The same conservation principles lead to both behaviors.

Yes, I sat down and did what you recommended so many times. I even explained (cited from Wikipedia) how the momentum is transferred to the rest of the atom ...

Yes, I understand that (and why) a single electron can scatter a photon, but tell me: the whole atom can? If not, why not? If yes, what decides if the atom as a whole or one of its electrons would scatter an incident photon?

 

[Dale]

the whole atom can?

Any charged particle can participate in elastic scattering. So as long as the atom is ionized it can participate. Inelastic scattering can involve even an unionized atom.

If yes, what decides if the atom as a whole or one of its electrons would scatter an incident photon?

Whether or not the electron is bound or free. If the electron is free then the electron scatters, if the electron is bound then the atom scatters. A bound electron cannot be treated independently of the rest of the atom.

For loosely bound electrons and high energy photons the difference between the two is small. I.e. the scattering is elastic for free electrons and inelastic for bound photons, but the degree of inelasticity may be small. In such cases you may see some references blur the line.

 

[DanMP]

... Whether or not the electron is bound or free. If the electron is free then the electron scatters, if the electron is bound then the atom scatters. A bound electron cannot be treated independently of the rest of the atom. ...

So, in scattering it is always about the electron ... We say that the atom scatters, just because the electron is part of an atom. This is very close to how I interpreted the absorption: the electron "receives" the photon but, because it is a part of an atom and momentum & energy are transferred to the rest of the atom, we say that the atom scatters/absorbs ...

 

[Dale]

So, in scattering it is always about the electron .

No, it is not always about the electron. It is only about the electron if the electron is unbound. I was pretty clear about that.

the electron "receives" the photon

As far as I can tell the word “receives” has no meaning in this context.

At energies below pair production there are three basic interactions: a photon does not enter but does leave (emission), a photon enters but does not leave (absorbed), a photon enters and a photon leaves (scattering). “Receive” is not one of them.

 

[DanMP]

No, it is not always about the electron ...

Ok, thanks for your replies.