Probing two-electron dynamics of helium in time domain via fluorescence channel

We theoretically study two-electron dynamics in a wave packet consisting of low-lying doubly excited states of helium, using an attosecond single-color XUV pump-XUV probe scheme. By solving the time-dependent Schrödinger equation in the hyperspherical coordinates, we analyze an ionization plus excitation process followed by a fluorescence emission. Pump-probe delay time dependence of the transition probabilities, and hence the fluorescence signal, is proved to unravel the bending vibrational motion of the two electrons with the nucleus at the center. Such a motion having an analog to that of a floppy triatomic molecule is the true feature of the electron correlation during temporal evolution of an intrashell two-electron wave packet.