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This question involves two cases: electrons bound to a nucleus and free electrons.

Bound electrons

Let's consider the hydrogen atom for simplicity. As far as I know, to be able to excite the electron, the energy of the photon should be in discrete values corresponding to the difference between energy levels inside the hydrogen atom. By the way in this link, the answer states that it is not the electrons that absorbs the photon but the atom in general, which makes sense to me (please correct or clarify if wrong).

The question is how long does the electron stay in that excited state, i.e. how quickly is the photon emitted back? Is it the same for all energy levels and all conditions like particle density (when many atoms together), temperature, presence of electric field, nucleus structure (neutron count) etc?

Free electrons

Again, according to the same link, free electrons do not absorb photons, which means they only undergo Compton scattering. Is this correct? If not, how long does it take for the photon be emitted back? Is the energy gain permanent?

Xfce4
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2 Answers2

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the answer states that it is not the electrons that absorbs the photon but the atom in general,

That is true, the system nucleus-electrons absorbs the energy. In the usual approximation of a nucleus at rest (due to its much larger mass a good approximation) one talks of the electron changing orbitals going to a higher energy level.

The question is how long does the electron stay in that excited state, i.e. how quickly is the photon emitted back?

The question is answered by the width of the spectral line by the energy-time uncertainty , although to get the correct number one must study the general broadening that can exist.

which means they only undergo Compton scattering.

This is correct, although I would include all kinds of scatterings, (Compton is high energy ).

anna v
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An isolated atom in a' excited state woul remain there forever. However, the atom necessarily interacts with electromagnetic field, other atoms, etc., which would cause it to reemit the photon. Some of these processes, such as spontaneous emission are independent on temperature and other conditions. Others, such as stimulated emission or relaxation due to collisions with other atoms can be temperature dependent.

Roger V.
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