Can electrons absorb a photon?

[DrClaude]

In lasers it's about emission of photons. You are sure that there is also an absorption line for this double transition?

Absorption and emission are symmetrical processes.

 

[DrClaude]

Transitions between states? That doesn't look like the atom as a whole is absorbing the photon and then use the energy to raise 2 electrons to a higher orbital ...

But the nucleus is still there, and its presence is required when writing the potential part of the Hamiltonian used to calculate those orbitals.

 

[DanMP]

Absorption and emission are symmetrical processes.

Well, they are, but still, it may be something similar to Sum-frequency generation, meaning that, in the copper vapour laser, 2 individual photons are in fact emitted by the (2 excited electrons in the) atom and then they "combine" into one.

Even if the above is/looks wrong/silly, I insist: you know for sure that there is an absorption line for this double transition? Maybe you can provide a link ...

 

[Dale]

I insist: you know for sure that there is an absorption line for this double transition?

By symmetrical processes he means that they both happen in a laser. So the laser he cited above demonstrates both absorption and emission.

 

[DanMP]

By symmetrical processes he means that they both happen in a laser. So the laser he cited above demonstrates both absorption and emission.

Sorry, but I'm not convinced. The laser in question may be "charged" both by one or by two photons, so I don't think that it proves without a doubt that one photon can cause a double transition. There must be an absorption experiment with that line present, in order to have an absolute proof. Do you know one? DrClaude? Anybody?

 

[Dale]

Sorry, but I'm not convinced.

I don’t think that is reasonable doubt. It seems to be a position based on ignorance of how a laser operates. If a material lases then it demonstrates both absorption and emission at the lasing wavelength.

Anyway, there are several promising sounding papers, but the ones I could find are all behind paywalls, so I couldn’t confirm their contents. You are welcome to do so.

https://doi.org/10.1063/1.435365

https://journals.aps.org/pr/abstract/10.1103/PhysRev.34.35

https://www.spiedigitallibrary.org/...es-in-active-media-of/10.1117/12.160510.short

http://iopscience.iop.org/article/10.1070/QE1980v010n03ABEH009986/pdf

I don't think that it proves without a doubt that one photon can cause a double transition.

Even so, you still have to contend with the clear proof that a double transition causes one photon. That clearly is not compatible with the electrons doing the emission. Since it is the atom doing the emission then why wouldn’t it be the atom doing the absorption? Particularly since the laws of physics governing the process are the same for both processes.

What is your reason for sticking with the “electron absorption” model? You seem to have a very strong attachment to it that I don’t understand. It doesn’t conserve momentum unless you introduce the rest of the atom. So why struggle to maintain it?

 

[DanMP]

... Anyway, there are several promising sounding papers, ...
...
What is your reason for sticking with the “electron absorption” model? You seem to have a very strong attachment to it that I don’t understand. It doesn’t conserve momentum unless you introduce the rest of the atom. So why struggle to maintain it?

Thank you very much for the links you provided. Unfortunately I can't see more than the abstracts, but maybe someone here can, and then help me/us to get a conclusive answer.

My main reason to insist on this (or any) matter is the fact that I need to understand what is really happening.

Yes, I preferred the idea that the electrons are absorbing/emitting the photons and that the momentum is transferred to the rest of the atom through virtual photons, but now I'm fine with your interpretation that the atom as a whole does the absorption/emission and transfers the energy to/from one or more electrons. Still, I want to know, if I can, which one is closer to reality.

LE: in both interpretations, overall, the atom does the absorption (/emission), but in mine one electron in the atom receives the photon, while in yours the photon is received by the atom as a whole. I wonder, if the atom as a whole receives the photons in absorption, why the Compton scattering is not on the atom as a whole as well?

 

[Herman Trivilino]

If my interpretation, that the electron in the atom receives the photon and absorbs its energy and momentum together with the rest of the atom, is the same thing with the mainstream interpretation that the atom absorbs the photon, it means that my interpretation is not necessarily wrong ...

Let's suppose an interaction between a photon of energy $hf$ and an atom results in the atom's energy increasing by $hf$. The photon disappears. We say it's absorbed, but in fact it ceases to exist, so that is different from the way a paper towel absorbs water. Without the atomic nucleus there would be no atomic energy levels. The potential energy is a property of the atom, not a property of anyone single component of the atom. It is the interaction between the components that gives rise to the potential energy, without that interaction there is no potential energy.

 

[DanMP]

... It is the interaction between the components that gives rise to the potential energy, without that interaction there is no potential energy.

My interpretation agrees with what you wrote. Even more, I think I mentioned this interaction (that takes place through virtual photons) when I cited from Wikipedia how the momentum is transferred to the rest of the atom.

 

[Dale]

My main reason to insist on this (or any) matter is the fact that I need to understand what is really happening.
...
Still, I want to know, if I can, which one is closer to reality.

Well, the usual model correctly predicts all of the observed phenomena, and (insofar as it differs at all) yours only predicts some of them, so I think that much is pretty clear.

I wonder, if the atom as a whole receives the photons in absorption, why the Compton scattering is not on the atom as a whole as well

Scattering and absorption are pretty different. 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.

 

[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.