Page "Atomic physics" Paragraph 9

In the event the electron absorbs a quantity of energy less than the binding energy, it will transition to an excited state.

After a statistically sufficient quantity of time, an electron in an excited state will undergo a transition to a lower state.

The change in energy between the two energy levels must be accounted for ( conservation of energy ).

In a neutral atom, the system will emit a photon of the difference in energy.

However, if the excited atom has been previously ionized, in particular if one of its inner shell electrons has been removed, a phenomenon known as the Auger effect may take place where the quantity of energy is transferred to one of the bound electrons causing it to go into the continuum.

This allows one to multiply ionize an atom with a single photon.