Far-infrared multiphoton ionization of lithium Rydberg atoms bypassing a Cooper minimum

Multiphoton excitation and ionization studies of the Li 23s state by 20 ps far-infrared laser pulses originating from the free-electron laser FELIX are presented. The two-photon ionization cross section of highly excited Rydberg s states is small, due to a newly discovered Cooper minimum in the bound-bound ns-n'p transitions. Instead of this lowest-order process, ionization proceeds via a remarkable multiphoton bypass starting with stimulated emission to a lower-lying state. The measurements are interpreted with calculations of the Cooper minimum and a Floquet analysis of the states involved in the bypass.