Two-photon double ionization of atomic beryllium with ultrashort laser pulses

We investigate the two-photon double ionization of beryllium atom induced by ultrashort pulses. We use a time-dependent formalism to evaluate the ionization amplitudes and generalized cross sections for the ejection of the 2s2 valence shell electrons in the presence of a fully occupied 1s2 frozen core shell. The relative contributions of the two-photon direct and sequential process are systematically explored by varying both pulse duration and central frequency. The energy and angular differential ionization yields reveal the signatures of both mechanisms, as well as the role of electron correlation in both the single and double ionization continua. In contrast with previous results on the helium atom, the presence of an electronic core strongly affects the final state leading to back-to-back electron emission even in the a priori less correlated two-photon sequential mechanism. In particular, a dominant pathway via excitation ionization through the Be+(2p) determines the profiles and pulse-duration dependencies of the energy and angle differential yieldsThis material contains work performed at Lawrence Berkeley National Laboratory supported by the US Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences Contract DE-AC02-05CH11231, and work at the University of California Davis supported by US Department of Energy Grant No. DESC0007182. Work at Cal Maritime is supported by the National Science Foundation, Grant No. PHY-1509971. Work at the Autónoma de Madrid was supported by the Advanced Grant of the European Research Council XCHEM 290853, the European grants MC-ITN CORINF and MC-RG ATTOTREND FP7-PEOPLE-268284, the European COST Action XLIC CM1204, the MINECO Project No. FIS2013-42002-R and the ERA-Chemistry Project PIM2010EEC-00751. We acknowledge computer time at the Centro de Computación Científica CCC-UAM and MareNostrum from Barcelona Supercomputing Cente