Five laser excitation of planetary atoms

The authors report the resonant five-photon excitation of both valence electrons of barium into highly excited states. Following two-photon excitation of one electron into a Rydberg state, the second electron is resonantly excited through a sequence of intermediate autoionising states. High-lying p and f states for the inner electron have been excited with three visible photons, the high-lying doubly excited states ('planetary-atom states') are parametrised in an independent electron MQDT picture. The advantage of this excitation method and possible extensions are discussed.