Double-hole states hollow atoms and triple excitation

6 Double-hole states hollow atoms and triple excitation 

A hollow atom is any atom for which at least one of the inner shells is completely empty. To produce a hollow atom by exciting only two electrons, the inner shell must therefore be Is2. The simplest hollow atom, and the one which has been known for the longest time is He in a doubly-excited state [310]. One can in principle consider partially hollow atoms, in which only a subshell rather than a complete shell is empty. 

The excited state is hollow, because the Is2 shell is completely empty. It turns out that the observed photoabsorption spectrum of the ls22s 2S — 2s22p2P transition exhibits a broad and asymmetric Beutler-Fano profile 

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