Study of shake-up states in helium by XUV-IR pump-probe experiments

The rapid progress in the creation of attosecond pulses paves the way towards time-resolved control and observation of ultrafast electronic dynamics. In a recent XUV-IR pump-probe experiment Uiberacker \emph{et~al.} [1] studied the ionization dynamics of shake-up states in Neon ions. The overall stepwise structure of the resulting double ionization yield as a function of the delay time between the two pulses results from incoherent tunneling of the excited shake-up states. However, recent theoretical studies [2,3] suggest that coherent effects play an important role as well. In addition, the influence of the IR field on the shake-up process might have significant effects. Since a full \emph{ab initio} treatment of Ne atoms in external fields is not feasible, we will study the process for helium where the full multi-electron dynamics can still be solved numerically. In particular, we will investigate the role of coherent effects, electronic interactions, and the presence of the IR field in the shake-up process. $[1]$ M.~Uiberacker et al., \emph{Nature} \textbf{446}, 627 (2007) $[2]$ A.K.~Kazansky et al., \emph{EPL} \textbf{82}, 13001 (2008) $[3]$ S.~Nagele et al., \emph{to be published}