# A photon absorption/emission by an atom

1. Jul 31, 2014

### -SJ-

Hello,

I am wondering about a photon absorption/emission by an atom.
We have been told at the university an electron is excited up in Bohr's model if an atom absorbs an energy in a form of photon. What cause an electron release an energy and falls down in Bohr's model? Does not an atom absorbs a photon in case of it doesn't have enough energy to cross a band gap?
Thanks.

2. Aug 1, 2014

### Drakkith

Staff Emeritus
When an electron absorbs energy, either by a photon or by other means such as a collision with another particle, it jumps to a higher energy state, as you know. But, this higher energy state is unstable, and after some amount of time the electron falls from the higher energy state to a lower one. It doesn't necessarily fall all the way back to the lowest energy state, as it can fall into several lower energy states if there are any available, but the key here is that this process is inherent. There's no known "mechanism" that causes this other than this inherent instability. It falls to a lower energy level simply because it can. (At least as far as I understand it. Someone correct me if I'm wrong)

Unfortunately I don't understand the last part of your post.

3. Aug 1, 2014

### -SJ-

That's another thing I would like to ask. Can be said how large amount of time in average this takes? I suppose it depends on a state orbital, is there any suitable equation?

I am sorry for my english, I am still working on my improvement. What I had in my mind was if can electron absorbs an photon until the photon has enough energy to get electron on upwards state.
I was wondering about it since that time and I've hunted out from my memory an electron absorbs and emits an photon normally in terms of elastic collision. But I am not sure with this matter: if a band gap is e.g. 1.3eV wide and electron obtain energy of 1.4eV it be excited on higher energy state and emits a photon with energy of 0.2eV, right?

4. Aug 1, 2014

### Drakkith

Staff Emeritus
I think so, but I don't know the equation myself.

I'm still not quite sure what you're asking, but I'll try to answer.

First, realize that each photon is completely absorbed. You can't absorb half of a photon. So all of the photon's energy goes to the electron. Also, a single electron typically only absorbs one photon at a time. If the intensity of the light is very high there is a small chance that it can absorb more than one at a time, however, but this is rare.

If the electron requires 1.3 eV to jump to a specific energy state, then absorbing 1.4 eV will send it to a higher energy state, if one is available. The band gap typically refers to an area where no states exist, but there may or may not be plenty of states higher than the bandgap that would allow a wide range of electron energies.

5. Aug 1, 2014

### -SJ-

Thank you for the answer. I think I can understand now. Thanks a lot.