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Summary form only given, as follows. Suppose both valence electrons of a magnesium atom are excited such that neither of the two electrons has enough energy to escape the ionic core. If they collide and exchange energy, one electron can decay to its ground state, whereas the other has gained enough energy to escape. This process is called autoionization. Normally the resulting electron emission decays exponentially in time. In a recent experiment we excited the first electron to a high stationary Rydberg Stark state. Subsequently, the second electron was excited with a laser pulse shorter than the orbital round-trip time of the first electron. In this special case the electron emission has a stepwise decay in which the step times correspond to the orbital round trip time and the step height to the probability to autoionize upon core passage. This peculiar ultra-fast emission pattern was observed using an atomic streak camera.