FAR-INFRARED MULTIPHOTON IONIZATION OF LITHIUM RYDBERG ATOMS BYPASSING A COOPER MINIMUM

Multiphoton excitation and ionization studies of the Li 23s state by 2 0 ps far-infrared laser pulses originating from the free-electron lase r FELIX are presented. The two-photon ionization cross section of high ly 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 bypas s starting with stimulated emission to a lower-lying state. The measur ements are interpreted with calculations of the Cooper minimum and a F loquet analysis of the states involved in the bypass.