Two-Photon Double Ionization of Lithium at FLASH

Two- and three-photon double ionization of atomic lithium by intense (I ≈ 10¹⁴ W/cm²) VUV light was investigated at the free electron laser in Hamburg (FLASH). Using a reaction microscope the recoil ion momentum distributions were recorded for two different photon energies of 50 and 59 eV. The former covers the case of non-resonant double ionization, whereas the latter gives rise to resonant excitation followed by double ionization. These reactions are of special interest as theoretical calculations for few-photon - few-electron reactions just become feasible for simple atomic targets.