Why do Excited States Decay?

Hi, Thanks in advance.. I read from one of your answer about "Why electrons move?" which is: "If they are only given some energy, but not enough to knock them loose, they will move from one orbital to another (say from the S-orbital to the P-orbital). But if there is no other electron in the lower-energy orbital, they will fall back down again. When they do, they release energy in the form of a photon (light). This is part of the concept that lasers are based on." 1. I wonder that why they fall back down? You wrote an answer in one sentence. But I want to hear more. 2. I think that if they are given a constant and exact energy, can they be holded in that state even if there are no electrons in lower states?
- Kadir YILDIZ (age 25)
Turkey

Here's that old Q&A for those interested: .

The answers to your two questions are very closely related. I can't seem to find our old answers, so here's a new one. Quantum mechanical states don't change physically in time if they are states with eact values of energy, called energy eigenstates.  When we say that say a 2P state is an enerrgy eigenstate we aren't quite telling the truth. The "Hamiltonian" expression for the energy of the atom involves not only the nucleus and the electron but also the electromagnetic field that interacts with them. 2P would be an eigenstate if the electromagnetic field were a purely classical thing. When the EM field is described in a proper quantum mechanical way, that 2P state turns out to contain components with slightly different energies. That means that it changes over time. So long as the atom is isolated in an infinite space, it ends up sitting back in the 1S ground state, with the extra energy carried off far away by the photon field. Since the initial state, when fully described, had a slight range of different energies, so does the final state. The 1S state really does have an exact energy, so the range shows up in the possible energies of those far-away photons.

Mike W.

(published on 11/24/2014)