Item

Abstract

In this work the first differential studies on two- and three-photon double ionization (DI) of lithium, have been performed at two different VUV-photon energies. Through the unique combination of a magneto-optical trap (MOT), a Reaction Microscope (REMI) and the free-electron-laser in Hamburg (FLASH), the momentum vectors of the doubly charged ions created, were obtained. These contain information on the electrons' sum momentum, as well as their mutual emission angle and energy sharing and hence on the correlated motion of the two ejected electrons. While at 50 eV photon energy a K-shell electron is ionized by non-resonant, simultaneous absorption of two photons, at 59.4 eV energy a one photon resonant transition (1s -> 2p) is followed by ionization through a second photon. In both cases it was observed that DI, i.e. the emission of the valence electron is either due to electron correlation or due to the uncorrelated, sequential absorption of a third photon. The comparison with results from non-perturbative close-coupling calculations is rather good at 50 eV, while poor agreement for the resonant process at 59.4 eV is found which most likely caused by an inaccurate description of the excited intermediate state. Thus, new insight in non-linear few-photon few-electron quantum dynamics in the VUVregimeis provided which is of paramount scientific interest, as well as of practical importance for many experiments at free-electron lasers.