Why Does an Electron Release a Photon After Absorption?

[Timothy Schablin]

After an electron absorbs a photon, it will move to a higher energy state. It then releases a photon and returns to its ground state. But why does the electron release the photon? Why does it not remain in that energy state? What forces it to return to ground state?

 

[vanhees71]

What you describe is spontaneous emission. You get it by quantizing the electromagnetic field.

 

[sophiecentaur]

What forces it to return to ground state?

One way to look at it is that there is a much greater probability of it ending up in the lower energy (unoccupied) state than in the higher state. Like if you build a pyramid of balls and you leave one out of a lower space. They may stay there for a while but they will eventually settle and only the lower energy states are occupied.
Statistics appears to create 'Forces" in many situations.

 

[rumborak]

Hah, I remember asking exactly the same question during our "material science" lecture in my EE Master's. Sadly, it was back then met with obvious ignorance from the lecturer.

To my understanding, the answer lies in the inherent probabilistic nature of quantum mechanics. The probability of emitting the photon is higher to emit than not; so, it's a question of "average time" until it will do so.

 

[HomogenousCow]

The excited states are not eigenstates of the full hydrogen+EM field hamiltonian and thus will evolve into a mixture of other states, with in general a nonzero projection onto the hydrogen ground state.