As examples, several Fano resonances in the extreme ultraviolet photoionization spectrum of neon are attributed to autoionizing states.
[2] Some are due to one-electron excitations, such as a series of three strong similarly shaped peaks at energies of 45.546, 47.121 and 47.692 eV which are interpreted as 1s2 2s1 2p6 np (1P) states for n = 3, 4 and 5.
When autoionization occurs, the np → 2s de-excitation provides the energy needed to remove one 2p electron and form the Ne+ ground state.
The same neon photoionization spectrum considered above contains a fourth strong resonance in the same region at 44.979 eV but with a very different shape, which is interpreted as the 1s2 2s2 2p4 3s 3p (1P) state.
In neon for example again, the excitation of triplet states is forbidden by the spin selection rule ΔS = 0, but the 1s2 2s2 2p4 3s 3p (3P) has been observed by electron ionization at 42.04 eV.