when an excited electron emits a photon what happens to the EM field of the electron
The electron can only emit a photon when it is accelerated. Nothing happens to the field due to the electron itself.
Isn't there some sort of recoil if the electron has super high energy say >1 GeV when the photon is emitted?
Actually, IIRC from another thread, its the changes in the fields due to acceleration that causes a photon to be emitted from a charged particle. I dont think the electron accelerates when changing from an energy level to the ground state because the electron doesnt travel through the distance separating the energy levels...it is not allowed.
That makes sense for a free electron in a synchrotron.
My understanding is that free electrons do not emit photons unless their paths are changed during the acceleration. Do electrons emit photons when they are linearly accelerated? If so, in what direction?
You are right, FREE electrons emit photons when accelerated. Since acceleration is a vector, changing direction OR increasing in speed in a linear path (or a combination of both) would all be considered as acceleration, and so in all cases the electron would emit a photon.
In the case of linear acceleration, is there a prefered direction for emission?
Thats a good question, im not sure but im guessing that it would emit the photon in the opposite direction of its acceleration. I guess to conserve momentum it would have to do that, but im just not sure, since this case involves a massive and massless particle.
When changing states in an atom, is it the electron which emits the photon, or the atom? I say it is the atom. So when you speak of an electron emitting a photon it must be a free electron.
The energy and direction of the emitted photon will be determined by the acceleration.
It's the WHOLE atom, so you are correct. This is because the energy eigenstates are the eigenstates of the electron within the potential of the atom. An electron, by itself, does not have the same eigenstates as it does in an atom. So it is the whole atom that participates in the transition.
Additionally, in cases such as photoemission, photoconductivity, etc., it is the whole SOLID (not just an atom) that participates in the transition.
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