# Energy from excitation

1. Jan 22, 2016

### Rahma Al-Farsy

Why doesn't an electron which has been excited not keep the energy gained and stay in a higher quantum level? Why does it prefer to emit radiation and return to a lower energy level?

2. Jan 22, 2016

### Simon Bridge

3. Jan 22, 2016

### Staff: Mentor

It can do so, and every process that is possible will happen at some point. Why should it stay in a higher level? This is true for isolated atoms as well.

4. Jan 22, 2016

### Rahma Al-Farsy

What do you mean by isolated atoms?

5. Jan 22, 2016

### ChrisVer

Well the transition from the excited state to the ground state has some probability to happen (and so some lifetime during which the atom stays excited).
Now the lifetime is pretty short for most of the atoms I know, and so they eventually end up in the ground state. If the lifetime happened to be relatively long, you would expect the atom to prefer being at an excited state (although I don't have such an example at hand).

6. Jan 22, 2016

### ChrisVer

isolated atoms: I think they mean the quantum mechanical system of the atom where there is no term in the Hamiltonian to account for the coupling of the atom to the radiation (EM) field... pretty much "classical quantum mechanics" (I guess)

7. Jan 22, 2016

### Simon Bridge

A system with only the atom in it... i.e. no possibility there is a state which consists of an atom and a photon.
The atomic model does not include the coupling with the EM field... did you read the link?

Doesn't that just change the wording of the question to "how come there is a non-zero probablity of decay?"
I think OP would like to know how this gets accounted for in QM.

Is key... the reason students may come to believe the lifetime of an excited atomic state is infinite is because they have usually been taught to work out the states by using a hamiltonian that does not include the coupling to the EM field. This is the level where students are taught that if a measurement of energy of the system gets you, say, the 1st excited state ... then all subsequent measurements of energy should return the 1st excited state (provided certain other things don't get measured in between energy measurements). So how does the QM learned so far allow that a subsequent measurement of energy may (eventually and inevitably will) get you the ground state?