What is meant by Excitation-Autoionization?

Question

I am confused and not sure what Excitation-Autoionization really means. In Dopita & Sutherlend's book 'Astrophysics of the Diffusive Universe' (2003, page 104) is written: 'At sufficiently HIGH electron impact energies, MORE THAN ONE electron of the target nucleus may be excited, leaving the atom in an unstable state, which is stabilized by the radiationless ejection of an electron, possibly followed by a radiative decay of the ionized atom back to its ground state. This process is known as excitation-autoionization.' On the other hand, in the Kaastra et al. (2008; Space Science Reviews, Volume 134, Issue 1-4, pp. 155-190) is written (Section 3.2.5.): 'In principle, if the free electron has INSUFFICIENT energy, there will be no ionisation. However, EVEN IN THAT CASE it is sometimes still possible to ionise, but in a more complicated way. The process is as follows. The collision can bring ONE of the electrons in the outermost shells in a higher quantum level (excitation). In most cases, the ion will return to its ground level by a radiative transition. But in some cases the excited state is unstable, and a radiationless Auger transition can occur. The vacancy that is left behind by the excited electron is being filled by ANOTHER electron from one of the outermost shells, while the excited electron is able to leave the ion.' Each individual explanation seem reasonable to me, but it seems to me that these explanations are different. Am I missing something? I would like to clarify what is meant by Excitation-Autoionization.

Answer 1

What they mean is that, when you transfer enough energy to an atom to excite more than one electron to an excited state you actually end up with enough net energy to ionize one of the electrons which eventually does happen.

For example let's say you excite $\text{He}$ to the $\text{2s 2s}$ state, since the energy of the atom is now greater than the energy of $\text{He}^+ + e^-$, eventually one of the electrons will get kicked out of the atom without any accompanying photons. This is what's known as autoionization. What this means is that states where multiple electrons are excited are actually not genuine eigenstates but instead meta-stable states.