The spectator electrons approximation

In principle, an inner-shell spectrum can be very simple if it happens 

The spectator electrons approximation (SEA) puts the most restrictive interpretation on the nature of a single electron excitation. What we suppose is that only one electron is involved in a transition, and that all the others act as spectators, i.e. that they do not change their properties in any way as a result of the transition. Although it is actually quite rare that this approximation works completely, it provides a good starting point, and can be considered as yet another kind of propensity rule (cf section 4.2), which reduces the number of strong transitions in a spectrum below the number allowed by the usual selection rules. 

It is worthy of note that the SEA is not useful in all cases. For example, if we consider inner-shell excitation from the d shell in an element with outer shells d10s2p, then the first excited state is d9s2p2, but the 

---