Electron-Electron Interaction & Photon-Electron Interaction?

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1. Apr 2, 2015

CAH

I learnt that photons can exite and ionize electrons in an atom, bring them to higher energy level etc. However ive seen a few questions on electrons bombarding electrons in an atom and exiting the orbital electrons to higher energy level.
Is this the same as the photoelectric effect when electrons absorb photons?
I know that electron positron interaction causes anniilation, so what is electron electron intreaction called?
v.v confused...

Thanks

2. Apr 2, 2015

CAH

Now i've just seen that the oribital electrons dont have to absorb all the incident electron when they bombard the atom? but photons have the be the exact amount???

3. Apr 2, 2015

ZapperZ

Staff Emeritus
That is what you have in your fluorescent lights! Electrons from a heated cathode is accelerated in an inert gas, and the electrons collide with the gas atoms, causing the gas atoms to be excited. When they decay back down, the transition emits light.

It is similar to the photoelectric effect, except that in this case, it is more similar to "photoionization", if you want to be really accurate about the terminology. (photoelectric is often reserved for photons hitting on metallic solids, not gasses).

Note that both photons and electrons can cause atoms to be excited to a higher energy state, or it can cause an electron to be completely liberated. It depends on the energy of the incoming photon/electron. The former is simply called an excitation (the excited electron never leave the atom), while the latter is ionization, where it leaves the atom with a net charge.

Zz.

4. Apr 2, 2015

ZapperZ

Staff Emeritus
You need to ask ONE question at a time, or else we will have cross-conversation like this.

That is why I said the process of photon-atom and electron-atom are SIMILAR, but I didn't say they are the same or identical. They are not. The physics is different, as in the conservation laws that are involved.

Zz.

5. Apr 2, 2015

CAH

Thanks so much!

6. Jul 27, 2015

D Tan

Now i've just seen that the oribital electrons dont have to absorb all the incident electron when they bombard the atom? but photons have the be the exact amount???

why is this?

7. Jul 27, 2015

blue_leaf77

Do you mean absorb photons? After being ionized by absorbing certain number of photons, an electron will become free. Free electron cannot absorb photon, otherwise the (relativistic) energy conservation law will be violated.

8. Jul 27, 2015

Staff: Mentor

Why do you think that?

9. Jul 27, 2015

blue_leaf77

As far as I know, when a photon interact with electron (or positron) it will be scattered, which we called Compton scattering. I think the violation of energy in the case of photon absorption can also be proved mathematically by using conservation of momenta and energy. By the way when you were doubting my statement, did you possibly think about the so-called "above threshold ionization"? In that sense, I think it's true that ionized electron can still absorb photons, that might be due to the fact that ionized state is not really free, instead it belongs to the (continuous) positive energy spectrum of the parent atom. So indeed, in that case the ionized electron can absorb certain few number of photons.

Last edited: Jul 27, 2015
10. Jul 27, 2015

Staff: Mentor

Yes, but that in itself doesn't rule out the possibility of other interactions.

Do you mean violation of energy conservation by itself, or violation of relativistic energy-momentum conservation? I missed the word "relativistic" in your previous post.

It's true that, if both the photon and the electron are on-shell (meaning they satisfy the relativistic energy-momentum relation $E^2 - p^2 = m^2$, where $m = 0$ for the photon), a single electron can't absorb or emit a single photon. But you can't really say whether it's conservation of energy, or conservation of momentum, that's violated; the root problem is really that the relativistic energy-momentum relation can't possibly be satisfied both before and after the interaction.

11. Jul 27, 2015

blue_leaf77

I think you are right, we can't really say which conservation is being violated.

12. Jul 31, 2015

Gaz

I don't think you can really say what is actually happening at all to be honest. I think what he is asking is why atoms only absorb certain frequencies of light (there emission spectrum). And it is a good question why does say a hydrogen's electrons only absorb red, light blue, blue and violet. And why would it emit the shortest wavelength of light when jumping the longest distance ? and the longest when jumping the shortest distance (n3 to n2). This makes no sense. In fact why does red and blue make violet and if that's true does it even emit violet at all or is it just red and blue? Oh and how does it even create a 656 nm Wavelength photon when its jumping a few pm at most?

Last edited: Jul 31, 2015
13. Jul 31, 2015

Staff: Mentor

Because their electrons exist in discrete energy levels, and they can only absorb photons with energies corresponding to the difference in two levels. You obviously know this since you refer to it later on.

I don't understand. The electron energy levels aren't separated by distances; they're separated by energies. We're talking about quantum states, not classical orbits.

The photon's wavelength should not be interpreted as a literal distance between wave crests. Again, we're talking about quantum states here, not classical waves.

(Even if we were talking about classical waves, your implied assumption that only something as long as one wavelength can create a wave of that wavelength is false. Antennas can be much shorter than the wavelength of the waves they receive.)

For a more detailed discussion of this, you should post a new thread in the Quantum Physics forum.

14. Jul 31, 2015

Gaz

Ah i was talking about the Bohr model of a hydrogen atom. with it's orbitals and jumping between them. I thought that was the standard model of how things worked. I think I get what you mean now =)

Last edited: Jul 31, 2015
15. Aug 1, 2015

Staff: Mentor

Nope. The Bohr model was superseded back in the late 1920's and early 1930's with the development of Quantum Mechanics.

16. Aug 1, 2015

Gaz

I see that but QM still predicts orbitals on a hydrogen atom exactly the same doesn't it ? I seen the first picture of a quantum probability wave or wave function in hydrogen the other day and it pretty much had orbitals. Apparently they knocked a bunch of electrons out of atoms and measured where they had came from or landed. Pretty interesting

http://physicsworld.com/cws/article/news/2013/may/23/quantum-microscope-peers-into-the-hydrogen-atom

Btw Drakkith that link you put in of the star map in the post about magnifying lenses is awesome.

17. Aug 1, 2015

Staff: Mentor

It does not. The Bohr model had circular and elliptical orbits and modeled electrons as being similar to planets orbiting the Sun. Quantum Mechanics is entirely different.

Thanks!