Attosecond cascades and time delays in one-electron photoionization

We present time-resolved ab initio study of the attosecond dynamics of electron-electron correlation during single-electron, single XUV-photon ionization of an excited helium atom. We identify multiple time scales in these dynamics, caused by the interplay of the initial shake-up excitation with the excitation cascades induced by the postionization interaction of the outgoing electron with the core. The time scales and the structure of these excitation cascades are directly linked to the time it takes the active continuum electron, lifted from a deeper bound orbital, to traverse the orbit of the passive electron residing in the outer orbit.