Electron ionization allows the observation of some states which cannot be excited by photons due to selection rules. In neon for example again, the excitation of triplet states is forbidden by the spin selection rule ΔS = 0, but the 1s 2s 2p 3s 3p (P) has been observed by electron ionization at 42.04 eV. Ion impact by high energy H, He and Ne ions has also been used.